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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).

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.

Nightbane Survival Guide also has rules for children characters.