Almost everywhere allows children to use firearms in appropriate environments with parental supervision. Many states also authorize children over X age (usually 12 or 14) to carry a gun while on their parent’s property (arguably for guarding cattle or whatnot— not really relevant to most kids today) with authorization from a parent.

The current status of the law in Missouri is insane— pretty sure even the Founding Fathers didn’t let children under drafting age just run around with guns.