DP, be careful about going too light with the younger kids (e.g., 9-10YO). Some kids cannot keep the batspeed up when the ball makes contact with the super-light bats. On video, I have seen the bat actually rebound backward from the ball at contact.
The batspeed that counts is the batspeed maintained through contact. So if the bat slows considerably at contact, it doesn't matter how much batspeed was generated prior to contact.
The matter at hand is transfer of momentum, of course.
Some youngsters are strong enough to not lose too much batspeed at contact with the super-light bats. Quite a few cannot do so.
Heck, the wood bats I used growing up were probably +2 or +3.