In real life weapons are similar to music instruments... no, not like that. What we mean is, no matter what someone's Weapon of Choice may be, using it is a finely honed art that they must practice, to the point where using another weapon might prove problematic.
In fictionland, however, if someone with an Iconic Item Weapon of Choice is shown as a child, has a child or meets a younger counterpart (a fanboy who wants to grow up to be just like them, perhaps) then you can expect them to own a toy or weapon with some perceived similarity to it.
The nature of this "Weapon Jr" and how functional it is can vary. It might be a literal toy version (wooden swords are popular, as are more high tech toys, like dart guns) or it might be an actual weapon that people wouldn't mind a kid having (Brats With Sling Shots, Slings, etc) which might actually be dangerous in Real Life (but not necessarily in fictionland). Whether it's just kids playing at soldiers, or children actually being forced into combat depends on the series.
Some stock examples of Weapon Jr. include;
- Bows: Slingshots are the most common one, however slingsnote and crossbowsnote are also popular.
- Guns: This is much more likely to be justified, since gun-like toys are extremely common, including Water Guns and Balloons as well as dart guns and popguns. Not to mention air rifles and BB Guns are only different in their underlying mechanism and power, and indeed are classed as actual weapons in many jurisdictions. In real life, there also exist fully-functional "youth" versions of rifles and shotguns in less-powerful calibers (.410 bore and .22 LR are among the most common), though one is very unlikely to see such guns used by kids in fiction (unless it's set at a Boy Scout camp or similar).
- Swords (and other blades): Wooden swords are the most common, followed by baseball bats, but a simple stick or any object longer than it is thin will also do. More violent children might instead use a smaller blade until they're older. Bonus points if they also use a trash can lid as a shield.
- Clubs and Hammers: More or less the same deal as swords. Pretty much anything smaller than the grown up weapon really. Maybe a pillow or squeaky hammer if they're really young.
- Staves and Polearms: A mop or any similarly shaped everyday object. Weapons that resemble tools (like scythes) can also have smaller versions of said tools.
- Axe: An actual axe, but of the domestic, firewood chopping variety.
- Flail: Yo-yos or toy nunchucks. For those who are cheap, a ball or a rock attached to a string also works.
- Whips: Riding crops, as demonstrated by a certain prosecutor during her early teen years.
- Instrument of Murder: While it might make sense to see the younger Spoony Bard training with their "weapon" at a young age, Rule of Funny demands that a simpler instrument be used (recorders and harmonicas being popular because of their being thought of as "children's instruments"). Toy versions work as well.
- Good Old Fisticuffs: This tends to be harder to "kiddyfy", but adorable oversized gloves, handwraps, safety gear or a simple martial arts uniform (especially if they don't wear one or customise it when they're older) can have the same effect.
- Grenades: Water Guns and Balloons can go here too, as can eggs or fruit, rocks or anything else that can be thrown. A more troublesome child might make use of cans of accelerant or fireworks.
- Armour: Generally this takes the form of either a costume or Improvised Armour made from scraps. Sometimes they might just wear an outfit which resembles it somewhat. Helmets tend to become a Bucket Helmet.
- Magic: Expect a future magic user to use a wand (actually magic or not), or own a young stage magician's kit (or see below). Especially if they come to carry a Magic Staff. Mops and umbrellas work for the latter as well.
- Superpowers: Finally, expect a child who will grow up to become a Super Hero (...or perhaps not) and has yet to manifest their powers play with or own some trinket that suggests them, such as a lighter for a pyrokinetic or "grabby thing" for telekinesis.
Specifically designed training weapons can also count if they're in the hands of children.
If this is used in a flashback, expect it to be milked for "Used to Be a Sweet Kid" points if they aren't an Enfant Terrible. If older versions of the protagonists using more advanced "weapons" show up then it's Generation Xerox or Future Badass. A video game might have your character wield them in a Justified Tutorial. In an RPG they might serve as Starter Equipment. May be wielded by a Kid Sidekick. This can be either a Call-Forward or Foreshadowing depending on when it's revealed who the child in question grows up to be. If children are actually being given weapons to played with (as toys or not) then it's My Little Panzer (although it still counts as this trope if the owner of it "upgrades" when they're older). Often turns up in A Minor Kidroduction.
- In Berserk, Guts used a full sized sword as a kid, as the mercs raising him didn't have any kid-sized weapons... but as an adult he also scales up his weapon and uses a BFS.
- In Mahou Sensei Negima!, small wands are used for magic training and as emergency magical foci. Chamo even describes them as beginner wands.
- In One Piece, Yasopp uses a rifle to snipe at his foes. His son Usopp uses a slingshot for the same purpose. Usopp seems to have no intention to graduate beyond a slingshot, however, instead upgrading the slingshot itself, notably with Skypea's superpowered seashells. It gets to the point where he can hit targets a normal rifle can't reach.
- Flashbacks in Magical Girl Lyrical Nanoha StrikerS show that Teana had a toy gun when she was a kid trying to emulate her older brother, which she still keeps as a Tragic Keepsake after he died during a mission.
- "Training lightsabers" can be seen in use in Attack of the Clones. The Expanded Universe elaborates on these lightsabers: they are built using the "kiddy" Bondar crystal which produces a merely zapping blade, rather than a deadly blade produced by the lethal, adult Ilum crystal. There's also a middle-ground crystal, called an Adegan Kathracite, which produces a cutting blade, but not as powerful as the real deal; Padawans tend to use these crystals in their sabers.
- In Moonrise Kingdom, much of Sam's "survival gear" consists of child-size versions of outdoors equipment, including a BB rifle.
- In the extended edition of The Hobbit: An Unexpected Journey, after a long prologue sequence told by old Bilbo on the eve of his going-away birthday party (featured in The Fellowship of the Ring), he recounts his first meeting with Gandalf, as a child at one of the Old Took's parties. Kid Bilbo hits Gandalf's legs with a wooden sword, foreshadowing his acquisition of his Elvish Shortsword, Sting, when he goes on his own journey instigated by Gandalf.
- Subverted with the Noisy Cricket in Men in Black. As a tiny, unimposing, toylike ray gun, J gets the impression it's this trope played straight when K hands it to him, but he soon finds out it packs one hell of a punch (and has one hell of a recoil too).
- Invoked (somewhat) in Captain America: The First Avenger: when Steve Rogers is being beat up in the alley, he uses a trash can as a shield to defend himself. Later in the film, of course, he gets the vibranium shield he's famous for.
- Borderlands's opening cutscene shows a picture of the Player Characters as children; Mordecai has a slingshot, Lilith has a bubble wand, Brick has a glove and Roland... has a map (although he does wear an approximation of the armour he wears as an adult).
- In Ocarina of Time Link gets the slingshot, and uses it until he can get the bow (which he can only use as an adult).
- Interestingly, Link's arsenal in Majora's Mask mostly consists of kid-sizednote versions of the same weapons he used as an adult in the previous game, like the bow and hookshot, showing how he's grown more into the hero role despite still having a child's body.
- Twilight Princess takes the trope even further, with Link (only an adolescent in this game) showing off his aiming skills with a slingshot, and his sword skills with a wooden sword.
- In Fallout 3, the tutorial has the player learn to shoot with a BB gun on their 10th birthday.
- In Kingdom Hearts which featured child versions of several Final Fantasy characters had sword-wielding Tidus use a wooden pole, and nunchaku-wielding Selphie had a jump rope. Wakka may count too, if you consider some of his spikier Blitzballs in FFX.
- Kingdom Hearts: Birth by Sleep had a teenaged version of Axel, when his name was Lea, wielding a pair of frisbees.
- Many of the Keyblade wielding protagonists used wooden toy swords before having a Keyblade, either for training or just play fighting. Terra, however, had a wooden toy keyblade.
- At the end of Metal Gear Solid 4: Guns of the Patriots, Raiden meets his son who uses a toy sword to perform the same moves he does.
- In Super Mario World, Bowser used his Koopa Clown Car in his showdown against Mario. In New Super Mario Bros. Wii, Bowser Jr. uses his own, miniature version of the Koopa Clown Car in his attempts to take out Mario (and Luigi, Blue Toad, and Yellow Toad).
- The Legend of Dragoon shows the armor variant of this trope briefly; The Hero Dart wears a red vest (similar to his red curaise) and his Love Interest Shana wears a white dress which is exactly the same shape as her armour (minus her Mini Dress Of Power). There is also a kid who's a fan of Lavitz in his hometown who wears a Bucket Helmet and carries a hoe (Lavitz wields Blade on a Stick).
- In Overlord II, the Overlad wields a little club that can't really hurt anything except annoying baby seals. As an adult, he can wield somewhat more lethal blunt instruments.
- In The Elder Scrolls V: Skyrim's "Hearthfire" expansion you can buy wooden swords for the children you adopt. They are also equipable weapons. You can even enchant them and make them Lethal Joke Weapon.
- Ace Attorney: Franziska Von Karma, the infamous whip-wielding prosecutor from Justice For All, favored a riding crop when she was 13 as seen on a flashback case in Ace Attorney Investigations.
- Overwatch's Halloween event has sprays that portray (almost) every hero as a child in a homemade costume going trick or treating, which also extends to their weapons. Examples include D.Va's mech being a big, painted cardboard box and Sombra using a can of silly string in place of her hacks.
- In an episode of Disney's Hercules series, in which Hercules initially scoffs at being forced to train with a basic training javelin rather than a more heroic weapon. But at the end of the episode, he uses a basic javelin to bring down the big bad guy (having been offered his choice of any weapon by Athena) and turns down the offer of a proper spear from an (actual) Amazonian Beauty.
- In the Looney Tunes short The Old Gray Hare, there's a flashback where Baby Elmer has a pop-gun which he fires at Baby Bugs. The episode also begins with an elderly Elmer obtaining a Ray Gun.