Here's what I use:
For the first 1/10th of your pre-adult life, you're two size categories smaller than your full size (so until 1.5 years, humans are Tiny). From that point on, until you reach 2/3rds of your starting age, you are one size category smaller than normal (so humans are Small from 1.5 years until 10 years old). From then on, you're of the usual size category.
Use the adjustments for size, but turn all bonuses into penalties as well (so a Tiny human has -8 Str, -4 Dex and -2 Con). Children suffer a -6 penalty on all mental stats while two size categories smaller than normal, reduced to a -3 when they are one size category smaller. When they reach their full normal size, they suffer only a -1 penalty on Wis until they reach the race's starting age (reflecting naivette and lack of experience).
Until they choose a class at the starting age, pre-adults have the stats of a 1st level commoner, but have no skill points (all skills are untrained), only 1 hp when two sizes smaller (increase to 2 hp when one size smaller and 3 hp when at normal size).
When you reach your starting age, you choose your class, based on whatever training you had, and substitute your commoner stats for your class's stats.