according to the rules, you can't have a "child" character.
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.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).
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?