DD swings a 30 in -12 crappy v grip bat. I want to buy her a new quality bat as recommended on this site but without demoing how do I decide what's the best size? I was thinking a 31 -11 or -10. Any advice?
The best advice I can give it to let her try some of her team mates bats during batting practice to see how she likes the different size and weights. Do some soft toss and front toss and check her swing mechanics and bat speed. If she is pretty strong for her size I would guess a 30in/20oz bat would be about right.
I would say by far the majority of 10u swing 30" bats, between 18oz and 20oz. If you are going into off season and plan on doing a lot of work, go ahead and go for the heavier size. If you are playing right now, a couple oz could really hurt a batting average for a while.