A typical (male human) 8 year old has roughly the following stats:
STR: 6 DEX: 6 CON: 8 INT: 6 WIS: 6 CHR: 8
That comes out to STR: - 4 DEX: -4 CON: -2 INT: -4 WIS: -4 CHR: -2. There's probably a tiny bit of 'rounding error' involved in making all the adjustments even but that's the D20 way.
As a child are size small rather than medium, with all the associated bonuses and penalties.
So long as they remain small, children also have the 'lithe' advantage giving them a +1 bonus on the following skills: balance, climb, jump, move silently, swim, and tumble.
Some may think that I'm being a little harsh here, particularly with the intelligence. First, these are 'realistic' attributes, not 'cinematic' ones. 'Cinematic' children are typically much more capable than there real life counterparts, particularly in the physical attributes. Note for example that the Star Wars rules will produce child heroes that are significantly stronger, faster, smarter, wiser, and so forth than average adults. Secondly, children simply don't have the experience that adults do, and the most charming and percocious of them don't have the broad knowledge and social understanding that average adults do. Also, while you may know children with very high IQ's, understand that children's IQ tests are often scaled differently than an adults, so that an adult with 160 IQ is much more intelligent and knowledgable than a child with 160 IQ. Even a child prodigy who may defeat average adults in virtually any academic challenge will have huge gaps in thier knowledge and reasoning elsewhere. You might want to simulate this by allowing some children to buy skill points at thier adult intelligence rate.
Most children will be 0 level 'apprentice' characters.