Well, I'll admit that in a perfect world, Garreth would be much more reticent, but the group appears to need a forcefull personality and so he stepped into the role. It's not that he's a great leader, more like the brains behind the scene.
The group itself reacts well to him, again partially in the name of group unity, but also because of unusual circumstances. The low charisma could represent the way standard children would react to a smart kid who told them all what to do. Some adults could also react negatively towards a mere child who presumed to be more intelligent than they are (because of course, he is

). So, I believe that Garreth's charisma score can still be justified given that he's only interacted with one NPC who he pandered to in order to pump him for information.
As regards the rules in general, the stat adjustments and pre-classes raise a few issues with me. Nothing bad, just my opinions and or questions. For example, what happens to them as time passes? Do they go away when you gain 1000 xp and gain your class level? Will Garreth suddenly become an 8 int wizard? Or do they stay until the character ages, and if so, when do they go away? When the character hits adult age, or before? What happens to my +4 int, does it suddenly drop one day, or does my score gradually raise according to age/adult age until I reach my base stat level when it stops growing (this seems to make the most sense, but when do I gain my charisma penalty back then?). I realize that your primary goal for this rule set are to create viable rules for playing children, and not neccesarily for playing characters from childhood into adult hood.
Also, the bonuses and penalties can cause wierd comparitive stats where the 18 base int roughhouser child has an int score of 5 while the 10 base int smart child has a 9 int. Your stat bonuses are chosen by the class you want to go into. An elven archer might want to be a fighter, but as a child will have strength and con bonuses. A brilliant general could be a fighter as well, but have a barely sentient intelligence level.
A possible thought I had is to not neccesarily lock children into a class based on their aptitudes. They pick up a class level at 1000xp in whatever class they like. This means instead of needing a child classification for each class, you just have six, one designed around each stat, like the d20 modern classes. Then you have a sort of background chosen for the child's home and family. This would differentiate between the rural, farm boy strong kid, and the urban, street bully strong kid.