A character type that's popular with children's mysteries and adventures that contain mystery elements.
The Kid Detective is part Snooping Little Kid, part Amateur Sleuth. This kid lives to snoop, but also sometimes to ask questions of the suspicious adults or those who may have seen or heard something important, and try to piece together what happened.
There are some notable differences between the Kid Detective and adult detectives, however, that make the Kid Detective a unique character. For example, while an adult can go straight to the authorities with evidence, a kid often has difficulty convincing adults of what s/he experienced. Hence the kid often has to let the adults know what's going on in a roundabout way, or expose it somehow. Sometimes the kid is in fact trusted by detectives, and can go to them directly.
Oftentimes the kid snoops and investigates without adults knowing. This presents many obstacles, such as finding out how to investigate not only without getting caught by the criminal, but without getting in trouble with parents or authority figures. If the kid does get in trouble, additional restrictions are placed, meaning the kid now has to sneak outside to do any more investigating.
The Kid Detective tends to be more adventure-oriented than the adult detective, meaning that there's often more emphasis on snooping and less emphasis on, say, interrogating or asking questions. Heck, kids can't interrogate suspects anyway, so asking questions to trick the suspect into revealing something is more likely. Also, due to the greater emphasis on snooping and sneaking, getting captured is a bigger threat, resulting in the kid getting Bound and Gagged and having to either break free, attract help, or be freed by a fellow Kid Detective. However, the kid detective is more often pitted against a Harmless Villain, thus avoiding the violence and genuine life or death situations that would surely be part of the adult detective's life.
This trope was a mainstay in the heyday of the Saturday Morning Cartoon like Scooby-Doo and its various imitations; it allowed for dramatic conflict, but focused on characters using their intelligence rather than force to solve problems.
If the Kid Detective comes from a detecting family, odds are they are a Born Detective. One who regularly wears a fedora and trenchcoat is probably trying to be a Hardboiled Detective.
Narutaki from Steam Detectives, son of Steam City's greatest detective and a child genius.
Fuyuki from Keroro Gunsou ends up acting in this capacity in a few episodes of the anime.
Death Note: The kids from Wammy's House seem to have shades of this, being a group of orphan geniuses raised to find the kid that will one day become L's successor. Near, especially, seems to fit early on, with his childish appearance and love of toys. He is, however, taken perfectly seriously by the authorities, if considered a tad unorthodox.
Detective Conan is a 17-year old de-aged down to seven. Since everyone thinks he's seven, he has to find ways of cluing adults in disguised as being a nosy seven-year-old. There's also Ayumi, Genta, and Mitsuhiko, the "Detective Boys." Although they're actually kids themselves, they aren't quite up to snuff as true Kid Detectives yet. They do try, though.
Majin Tantei Nougami Neuro is a subversion. Although Yako is supposed to be the leader of the detective agency (and she is billed as such), Neuro is the one doing all the detective work and is using her as a front. Later on, it is hinted that Yako is actually a semi-competent detective on her own, though.
The aptly named "Sleuth Brigade" from The Voynich Hotel try to be this, but fail frequently, as they are constantly bogged down by their own issues and the usual weirdness doesn't make any attempt to conceal itself.
Tubby from Little Lulu occasionally acted as one, whenever Lulu needed his help with a problem at her house.
Cat Curio in Hack Slash is a deconstruction when she's first introduced, showing what it would really be like for a preteen to stick her nose in murder cases. She's extremely observant and ingenuous, but the police find her anything but helpful, she seems to have no concept of danger, and her snooping almost gets her killed.
Instead of dying, she was put in a coma for 13 years. When she awakes, she returns to her work as a detective and starts her own agency, but obviously she still has the mind of a 12 year old autistic girl.
Parodied with Young Jack Black in Viz, who is an extreme-right-wing Jerkass who often inflicts Disproportionate Retribution on people who, in some cases, weren't even doing anything legally or morally wrong.
The Facing The Future Series features Mike Harris, a boy at Elm Street Middle School who likes to snoop on things and may be on the road to learning Danielle's secret.
Turnabout Storm: Apple Bloom tags along Twilight in her investigation at one point. She doesn't do much proper investigation, but she does make Twilight stumble upon several pieces of evidence, and has a moment of useful Genre Savvyness.
The comedy Mystery Team is a Parody of this character type. The main characters are 18-year olds still stuck in their crime-solving days and are Man Children. To increase their actual cred, they take on a little girl's case, which is looking for her parents' murderer.
The plot of the movie Clubhouse Detectives about a group of kids who work together to solve the murder of an opera writer's wife.
Natalia in the Grey Griffins series carries a notebook to write down clues and figure things out. She sometimes snoops around on her own and tries to discover things, separate from her group of friends. Through most of the story, she is more of an adventurer than a snoop, though she still is often the one to piece together clues.
Cam Jansen is an elementary schooler who uses her Photographic Memory to remember exact details and use them to solve crimes.
The same author has a boy character named Jeffery Bones with his own series.
Encyclopedia Brown. Notable here is the fact that his father is a police officer who knows of his son's activities, and is somewhat embarrassed that the smartest detective in town is a fifth-grader.
The Hardy Boys, who, at 17 and 18, are two of the oldest examples of "kid" detectives out there, though they were originally 15 and 16.
Nancy Drew: Nancy is a young but brilliant teenage girl solves mysteries with the help of her two best friends and her understanding father.
The Dana Girls.
Trixie Belden (and the rest of the Bobwhites) are teen detectives in the Hudson Valley. Thanks to Trixie's rich best friend Honey Wheeler, they frequently go on trips and conveniently solve mysteries wherever they go.
The ten-year-old members of the McGurk Detective Agency, led by the clever if perhaps overly-impetuous Jack P. McGurk.
Enid Blyton's Famous Five. And Secret Seven. And Five Find-Outers. And others.
Paul-Jacques Bonzon's Les Six Compagnons, the French counterparts of Enid Blyton's Kid Detective series.
Alan Coren's Arthur The Boy Detective, who lives at 221A Baker Street, and constantly shows up his downstairs neighbour.
Justin Richards' The Invisible Detective novels are about a group of kids who claim to be "Baker Street Irregulars" to the non-existent Brandon Lake, because no-one would take them seriously as detectives themselves.
A darker, deconstructed version also appears in Newman's "Clubland Heroes" with Richard "Clever Dick" Cleaver; he's an off-the-scale detective genius who, unlike the more pleasant and engaging Richard Riddle, is also a snide, stuck-up and humourless little snot. And then when he appears in "Cold Snap" following the ignominious end of his child-detecting career, he's let bitterness warp him into a genocidal maniac.
Jill Pinkwater's The Disappearance of Sister Perfect has Sherelee Holmes, who, after deducing that her runaway sister has joined a cult, poses as a rich teenager several years older than herself and infiltrates the organization.
Walter "Ramses" Emerson was an example (a master of disguise among other things) until, over the course of the series, he grew up. He is the child of Amelia Peabody and Radcliffe Emerson, a Battle Couple pair of Egyptologists and incidental detectives in a series by Elizabeth Peters. Ramses has since grown up, gotten married, and produced his own frighteningly precocious children. His mother feels it serves him right.
Note that in the following passage, rescuing his parents, Ramses is about eight or nine:
"Now, Mama, Papa, and sir," said Ramses, "please withdraw to the farthest corner and crouch down with your backs turned. It is as I feared: we will never break through by this method. The walls are eight feet thick. Fortunately I brought along a little nitroglycerine—" "Oh, good Gad," shrieked Inspector Cuff.
Played with in an Esp Mcgee book where the book's kid Watson decides to personally visit the home when the kid suspect is out to get some information. As it happens, he barely manages to excuse himself when the suspect and his menacing father arrive home early. Afterward, the terrifying experience weighs so much on him that he confesses to his parents what he was up to. His alarmed parents give him a firm lecture about taking such risks, but playfully then suggest that since he's done it, he might as well contact Mcgee to give his report, which proves to crack the case.
The Shirley Holmes stories depict the exploits of a certain detective's sleuthing teenaged sister. (Not to be confused with the TV series, which they inspired.)
Astrid Lindgren's Kalle Blomkvist, translated into English as Bill Bergson. He's thirteen to fifteen in the books and played quite realistically. He has a hyperactive imagination but enough analytical skills and knowledge of real detective procedures (like taking fingerprints, and even chemically detecting arsenic) to find real evidence against them when he's finally confronted with real criminals. Together with his friends, he's also resourceful enough, and suitably experienced in pretend warfare of sorts against their friendly rivals, to make it through sticky situations involving dangerous adults.
The Great Brain series by J.D. Fitzgerald — the Great Brain Himself, when there's a serious crime to solve. Subverted in that when there isn't, the Great Brain is more likely to be swindling other kids out of their pocket change.
In the earlier Harry Potter books, Harry, Ron and Hermione fit this pretty well, and this element runs through all the books.
Sugar Creek Gang: These evangelical Christian-themed books feature kid detectives having wilderness adventures. And a lot of preaching.
TKKG falls into this category, combining it with a Five-Man Band. Also the problem with the authorities is not that big because the father of one of the kids is with the police. And since in later books they also have a reputation for solving crime.
Agatha Christie's Crooked House: Twelve year old Josephine investigates the murder of her grandfather, using her naturally snoopy nature to provide clues that the outsiders to the family never manage to find. However, it then turns out she's the murderer, having decided to kill her grandfather over his not getting her ballet lessons. She had decided to investigate the murder to get further attention from her family and the police. The subversion naturally comes from the fact that readers had been led to believe she was a genuine Kid Detective, before we find out what was really going on.
Enola Holmes: Enola is the much younger sister of Sherlock Holmes. At only 14, she is able to live independently, establish a successful detective business through a lot of misdirection and elude her brothers' best efforts to capture her until they wise up about her.
The Stanley family in Zilpha Keatley Snyder's loose series of books about them. The concept is partly deconstructed with Janie's Private Eyes, where 8-year-old Janie ends up causing nothing but headache and trouble with her investigations.
The Sweetness At The Bottom Of The Pie: Flavia de Luce is a great example. She also has some Mad Scientist tendencies that lead to her being an eleven year old expert on poisons.
Harriet from Donna Tartt's The Little Friend, who plans on solving the decade old case of her brother's murder. Deconstructed as she has absolutely no bearing on what to do - as nobody wants to discuss the murder as it practically destroyed the family - and is acting like her favourite fictional heroes, getting herself and others into unrelated trouble. She goes entirely on hearsay and jumps to the utterly wrong conclusions while ending up severely injuring an old woman by throwing a poisonous snake into her car (believing it to be driven by the murderer), accidently killing one of her brother's friends (believing him to be the murderer with no evidence at all) and causing herself serious bodily harm, while "killing" said friend. She did accidently stop illegal drug dealing without knowing that it was drug dealing.
Lasse and Maja from Swedish author Martin Widmark's Lasse-Majas Detektivbyrå-stories (20+ books, a TV-serie, The Movie...). They have no trouble convincing the local chief of police of their findings - while well-meaning and competent, he is still unable to solve any crime without their help, so...
Inspector Tearle appeared in five books in the late 60s and early 70s. He and his sidekicks, his athletic sister and his Watsonesque best friend, solved cases from a treehouse headquarters.
Brains Benton and his Watson, Jimmy Carson, solved six cases in the late 50s and early 60s. Brains himself was a strong Sherlock Holmes expy, and possibly (for his age) even a better scientific detective.
Eugenie Sandler PI. was an Australian TV series about a teenage girl whose father is private investigator. When he disappears, she is forced to turn detective in order to find him.
Naoto Shirogane of Persona 4. Dissected, as well: the police force hates having to call Naoto in on the serial murder case, and she's passed herself off as a boy for years because of how male-dominated the police are. Her Shadow taunts her over how mature she tries to act to get past the "young detective" bit - it's worth noting that her Shadow flip-flops between overly mature dialog and crying like a baby.
As each of the characters in Guilty Party is based on a classic detective archetype, Rudy (AKA Kid Riddles) represents this particular trope. His cousin Ling-Ling is one too, but she's a teenager and skews more towards being a Nancy Drew-alike.
Parodied with the Mystery Solving Teens from Hark! A Vagrant - unlike most teenage detectives, they act like typical teenagers, meaning they rarely do anything relevant to the case.
The kids from Bad Machinery regularly deal with crimes and mysteries, mostly with a supernatural spin.
This trope is somewhat subverted in "The Case Of The Forked Road", where after the kids consider all sorts of devious ways to fool railway men into pulling the levers to switch a train to a different track to prevent a crash, or to get the men out of the way so they can pull the levers themselves, Shauna finally takes the direct, straightforward approach as an adult investigator might, and tells the men that the track has been sabotaged and they need to flip the switch; and they believe her and take immediate action.
The series also has the variant that the local sheriff is well aware of how skilled the kids are and regularly consults with them (He must have severe budget constraints for investigations if he has to turn to a group of teen amateurs, albeit skilled ones).
The Simpsons has done this numerous times with Bart and/or Lisa. The first such instance is when Krusty gets arrested for armed robbery but some sleuthing discovers that it was actually Sideshow Bob trying to frame Krusty. Nelson also takes it up later on, with Bart and Lisa being the suspects.
Numbuh 2 of Kids Next Door occasionally acts as one, complete with noir film-esque narration.