My DD is 10 and hit a grand slam almost to the fence last weekend with her 30/20. She's tall for her age though only about 85 pounds. I was just talking to DH about what a strange animal hitting is. On our first 10U team there was a girl who was teeny tiny and one of the best hitters around. She must have the most amazing hand eye coordination. The kid NEVER strikes out and after her first few tournaments was hitting for power too, this tiny thing with her 30/19 bat (I love this child and root for her and the rest of her team all the time...but I can't help but hope for the day DD becomes one of the few pitchers who can strike her out!). Then sometimes you'll see a girl come up who's really big and you back up all your outfielders and she can't get it out of the infield.
Decent swing mechanics for generating high bat speed, and the athletic coordination to consistently square up the ball on the sweet spot is a wonderful combination.