according to the rules, you can't have a "child" character.
You can choose or randomly generate your character’s age. If you
choose it, it must be at least the minimum age for the character’s
race and class (see Table 6–4: Random Starting Ages).
14 is the minimum age, and that's for a half-orc. but hey, it's a game. whatever makes it fun is great, so long as everyone agrees to it.
if you're just looking for ideas to kick around with, maybe drop a stat point in everything but charisma until they hit the minimum age for their race?