leveling up Children Player Characters.

[drewkitty ~..~]

this might be delt with in ether the Systems Failure book or the Chose Earth book. Another place you might look are BTS books (1 & 2).

[Cinos]

You could slap on a flat EXP Gain reduction (slower advancement and thus, slower power as a result), something to the tune of -50%. You could also cut the bonsues they gain from skills / W.Ps until they reach the proper age. Either that or start them at a sort of Negetive level (i.e skills are cut, bonsues are cut, fewer starting skills), and make them gain the levels that are sort of 'assumed' when characters start off.

[Nekira Sudacne]

Nightbane Survival Guide also has rules for children characters.