Typically the youngest member of a group, the Tagalong Kid is usually the Hero's or the Chick's younger brother. He gets to save the day about once per season, but in the worst case scenarios he'll end up being The Load or Designated Victim in every other episode.
If the kid is part of The Team instead of just a tag-along, the kid might sub in for just about any role but with the proper tropes behind him or her.
If given combat ability, they tend to use their youthful energy and small size to fight as a Fragile Speedster. Alternatively, they may use some form of ranged weaponry to strike from a safe distance as a Glass Cannon. Slingshots and other throw weapons are more common than relying on archery or firearms.
Get Backers offers a subversion in the form of 14-year-old MakubeX, who in flashbacks is shown to be something of a tag-along to Ginji. But upon the latter's departure from their hometown, he goes on to become its ruler and has several adults working for him. Some of the said adults "tag along" for their own purposes, while the others are devoted followers who would go to the ends of the earth for him.
Clare is Teresa's Tagalong Kid in Claymore, and Raki becomes hers.
Raki gets one himself after the Time Skip, citing Clare's treatment of him as his motivation. Of course his happens to be Priscilla, the single most powerful Awakened Being in existence.
In Yotsuba&!, Yotsuba is everyone's Tagalong Kid...when she's in the mood for it.
Goku of Saiyuki has the personality and relationships of a Bratty Half-Pint down pat — but he's the closest thing the series has to a genuine hero, and it's his relationship with Sanzo/Konzen that is the impetus of...pretty much everything that happens in the plot.
Alphonse Elric and May Chang in Fullmetal Alchemist are barely teenagers, but they are both very skilled fighters, and Alphonse happens to be a soul attached to a giant suit of armor, making him among the largest and toughest characters.
In fact, by the time FINAL rolls around, Mamoru has arguably Taken a Level inKid Hero; when he reappears late in the series, his purification abilities have gained some real destructive power, he gets his own Final Battle with one of the Sol Masters, and his ultimate Determinator moment in said battle empowered the rest of the team's G-Stones when the good guys were on the knife's edge of defeat. The series would almost certainly have had a Downer Ending if Mamoru hadn't been there and hadn't been as Badass as he was.
July in Darker Than Black, first to November 11 and April and, in the second season, to Hei. However, despite the Extreme Doormat attitude, he's actually an important member of the group; as a Doll, he can do reconnaissance without anyone having to actually get in the line of fire.
Nieble from Rave Master is introduced as a child Sieg knows who instantly starts following him around. Once Sieg passes away he ends up with Belnika at all times.
Rua from Yu-Gi-Oh! 5Ds is this to the main team. He's the same age as his twin sister, Ruka, but not having any Signer powers he tends to default to this role. Much later on he Took a Level in Badass and becomes a Signer himself.
Son Gohan of Dragon Ball Z starts off as this in the early arcs, only to invert it in the Cell arc when he becomes the most powerful character in the story. After Gohan grows up, Goten and Trunks share the role.
Capella of 07-Ghost becomes this after Teito and Frau rescue him from slavery. He later leaves the group when he is reunited with his mother.
In Digimon Adventure, Takeru and prior to Hikari joining the team, his Digimon Patamon only evolved to Adult and fought, twice. Once Hikari joins the team he becomes a lot more active. Ironically, Patamon's evolutions were the most powerful of the team, which is why they didn't appear much (He was always the last to evolve — HolyAngemon didn't debut until 3 episodes before the end!).
In Digimon Tamers we had Jianliang's little sister Xiaochung. Her partner Lopmon didn't get too much action in the final arc partly because of this; the other bits being the Spotlight-Stealing Squad.
JoJo's Bizarre Adventure loves this trope. Part 1 has Poco, a kid who is rescued from vampires by the heroes. Part 2's is Smokey Brown, a kid who is rescued from a Dirty Cop. Part 3 features an unnamed girl who is rescued from an assassin sent by the Big Bad (seeing a pattern here?). Parts 4 and 5 finally break the trend with Koichi Hirose and Narancia Ghirga, respectively, who actually have some combat skill. Part 6 then returns to form while subverting it with Emporio Alnino, a kid hiding out in the prison where the story is set. His ability is arguably the weakest in the entire series, but due to circumstance he ends up being the one who finally defeats the arc's Big Bad. Part 7 finally breaks the trend; the entire cast is composed of adults and no-one is anyone's tag-along.
Edward is this in Cowboy Bebop, as nobody actually intended for her to join the crew after she helped them out once. She also fills the role of The Smart Guy.
Wendy of Fairy Tail started to become this to series' resident Five-Man Band Team Natsu shortly after joining Fairy Tail, and while she is a de facto member now, considered equal with the others in-universe, she is still largely a Tagalong Kid to the readers. She has since graduated to being The Chick, sharing this role with Lucy, who also spends time as The Smart Guy.
Doctor Tenma from Monster has Dieter for a large chunk of the plot who insists on following people.
Ako Shirabe/Cure Muse in Suite Pretty Cure ♪ turns out to be this. She acted as a masked Aloof Ally most of the time, but pretended to be older and taller than she is. Despite her age, she's very smart and Genre Savvy. Of course, she's similar as strong than other Pretty Cures. She's even the first to become a Pretty Cure. However, she the only one who has not a Belltier or another rod.
The Selfish Trio from Doki Doki Pretty Cure has Ira, a villainous example. Unlike his teammates, he relies on brute strength instead of smart plans. Even the Monsters of the Week he summons are usually dump and simple.
Sealand of Axis Powers Hetalia usually ends up as this when he tries to involve himself in the affairs of the real nations. He generally gets chased off because nobody is willing to recognize him.
Isabel from Red Hood and the Outlaws, the stewardess whom Red Hood takes on a date, is accidentally teleported with the rest of the team onto the Tamaranean space ship. She reacts as well as you'd expect.
Scott Larson in Midnight Madness He was picked up by one of the teams when he was trying to run away. He ends up saving the day twice, once by enabling the team to beat a video game, and also by using his retainer to restart a stuck elevator.
It seems to be a "rule" of the Jurassic Park series that the main characters must always be accompanied by at least one kid. Jurassic Park kids tend to be The Load most of the time, but will nevertheless probably have a Chekhov's Skill which will prove useful about once in their film.
Kid from Six-String Samurai. Yes, Kid is actually his name.
Chekov in the 2009 prequel/reboot of Star Trek is only seventeen, and thus some of the other characters (who are mostly very mature individuals in their twenties) seem to regard him as this. Which he is, to an extent, but he's also pretty clearly gearing up to be The Smart Guy as well.
Defied in Iron Man 3. Tony Stark brushes away Harley in his attempts to bring him so he can keep him safe from danger.
Gavroche in Les Misérables is this to the students at the barricade. He is killed after a few pages.
Adam's younger brother James in Shatter Me who he and Juliette have no choice but to take on the run with them to stop Warner from harming him.
"Runt" in Dinoverse is the little brother to Zane's host. Zane refuses to leave the herd at first until he's kicked out to fetch Runt. Then he encounters the other formerly human characters, and he can't leave the little guy alone, so...
The Brady Bunch: Cousin Oliver was never given much to do except be Bobby and Cindy's sidekick.
River Tam on Firefly. She has a much bigger role in the movie.
Alexis Castle on Castle. She also doubles as The Smart Guy/ Teen Genius and provides lots and lots of helpful insight to her Man Child father. And when she shows up at the police station, everyone minds their language.
Vaan in Final Fantasy XII — a bit unusual in that he's the viewpoint character. Gameplay-wise (and keeping in mind that there's a large amount of Gameplay and Story Integration) Vaan is out and out the best character on the team, assuming you use him at all. This would seem to imply that Vaan is a monster killing machine the team fortunately has lying around.
Later games show Vaan has gone into business as a god-like thief and have him be a skilled enough fighter to hold off a Physical God for a long time. So, the FFXII team probably didn't know what they had on their hands. As incidental as Vaan's inclusion on the team maybe, he's probably the single most powerful weapon at their disposal, if not instrumental to their success.
Penelo went through a similar change. She played an even less of a role in Final Fantasy XII than Vaan did and her only purpose for tagging along was to make sure Vaan was staying out of trouble since he was the only family she had left. Her role was expanded upon in the spinoffs, but she's still mostly portrayed as Vaan's partner who make sure he doesn't cause too much trouble.
In The Elder Scrolls IV: Oblivion, winning all Arena matches saddles you with the "Adoring Fan", a small boy who follows you around gushing about your deeds.
Mission Vao and Zaalbar from Knights of the Old Republic. Mission is more obvious, being Hot-Blooded and a Fragile Speedster (her dexterity is very high), with a bit of the "genius" as her true strengths come from being more well-rounded in the skills department than the droids. She's also higher on the Karma Meter than the party's Jedi (tied with Carth, surprisingly). Zaalbar is definitely The Big Guy, who's actually marginally higher on the Karma Meter than Jolee, but still light-sided. He doesn't really appear to be a kid (Wookiees age differently than humans), but his interactions with Mission, and the fact that he's constantly referred to as "young" mean you likely have two adolescents along for the ride.
By exploiting a glitch, you can get six-year-old Bumble to tag along after you throughout Fallout 3.
Hope of Final Fantasy XIII is a civilian that was unlucky enough to be caught up in the Purge. He stubbornly follows Lightening for the bulk of the game. Unlike some kids, he's incredibly useful, because he has the highest magic stat in the game.
Aila to Geddoe's mercenary group in Suikoden III, at least in the mercenaries' opinion. She ''was a warrior in her home village, but still young and naive compared to the rest, and at the end of the game Geddoe doesn't let her join the group officially, but he does let her stay around, making her an official Tagalong Kid.
Riki from Xenoblade. All the other party members have a good reason for wanting to fight the Mechon, be it to avenge or to rescue loved ones. Riki suffered none of these problems, and is pretty much roped into going along with them. Of course, that's not to say he's just The Load, far from it!
Ellie from The Last of Usdeconstructs this. She was born into the Crapsack World that the U.S. has become, so she's effectively known from birth that she needs to pull her own weight and be just as useful/capable as adults in order to survive. She also knows not to be a Neutral Female, helping out Joel when and how she can.
Dr McNinja: Do you think I run around with a twelve-year-old boy just because I like his inferior grasp of girls and higher level math? Do you think I left him with my psychotic parents because I wanted him to die? No, you undead pale ponce! Gordito is the effing badass kid.
In Rescuing Dara, Dara has to come along with the team for their plan to work, and she does help sometimes, but she still is this trope.
Jade, from the Whateley Universe, looks 11. She's actually 14 and most people who meet her think she's the Tagalong Kid and The Load of Team Kimba. She's not actually.
Matt, Leo and Mark in the first season of Omega Guardians. Andy could also count as well, even though he was treated an equal part of the team depending on the episode.
In Worm, Theo Anders, son of the Nazi supervillain Kaiser but who lacks powers of his own, is generally brought along by his stepmother Purity with her group, the Pure, because he serves as a caretaker for his half-sister Aster.
Instead of sending her on a plane to Hong Kong, the Runaway girl follows the Joestar group in Vaguely Recalling JoJo.
Ray Thompson, in the episode "Legends" from Justice League. For the most part, he's a useless sidekick who cheers on the heroes featured in that episode, the Justice Guild. The twist here is that the Guild and in fact the entire city are an illusion created by Ray, who in reality is a mutated psychic child living out his fantasies by warping the world to his whims.
Jan and Jace in the original Space Ghost cartoons essentially were two kids who went along on Space Ghost's adventures.
Jonny Quest himself, in the original Jonny Quest cartoon. When he aged up to teenhood in the later series Jonny Quest: The Real Adventures, he was much more actively involved in the adventures. Though he got into many scrapes as a kid, too.
Pete, Dr. Quinn Darien's nephew on The Godzilla Power Hour, although he has the benefit of being the main means of talking with Godzooky and, by proxy, Godzilla.
Spinner in the Clutch Cargo cartoon series. Why did Clutch need to look after a little boy anyway?
In My Little Pony: Friendship Is Magic, Spike the baby dragon serves as assistant and de facto little brother to Twilight Sparkle, and frequently comes along on the ponies' adventures. Downplayed in that the ponies rarely bring up his age and he's shown to be just as capable as the adult ponies, except for one instance in which he had to miss out on an adventure because he was too young to stay up all night.
Played cutely straight in the openings of seasons 1-3, where Twilight easily jumps out of the balloon basket, but Spike has to climb out and run after her.
Max LeBlanc in Night Hood. Might also qualify as a child's version of a Distressed Dude, due to getting into trouble every other episode and having Lupin come and save him. One episode revolved around him getting kidnapped and Lupin had to rob Fort Knox in order to get him back.
Let's not forget the other tagalong of the Mario Bros., which is Oogtar in Super Mario World.
Steven Universe is an odd example where this character is the protagonist—it centers around a half-magicalkid who lives with the Crystal Gems, a team of magical guardians whom his mother used to be a part of.
Angelina Ballerina has Henry, Angelina's younger cousin, as the tagalong in the original books and the 2002 series. Polly, Angelina's little sister, takes over this role in the 2009 CGI series.
Reality Is Unrealistic — The Iwakura Mission of Meiji-era Japan was a Japanese tour of the world intended to help the country modernize, so that it would be able to compete with the Western nations. Because it was a mission of modernization, almost all the passengers were adults, who were sent off to study at Western universities so they could use that knowledge to improve their own country. Yet despite this, one of the passengers was Tsuda Umeko, a six-year-old girl. She was dropped off in the United States and taken in by an American couple to be educated there, but the fact that she was there at all is bizarre considering the mission's purpose. (It is possible that this is the reason there are so many examples of this trope in Japanese works, but more likely than not that is just because of Japan's cultural obsession with cuteness, which ironically did not exist in Tsuda's time.)