A current 10U girl I'm scouting uses one in 31" length. It seems to have good pop/exit speed and she has launched a few bombs with it. She's a big kid, though, and plenty strong.
I also know of a smaller 9-year-old who uses one. Her mom is a former DI player and really knows her stuff. They like it just fine and actually prefer it to other similarly-priced bats like the Vendetta and Combat Lisa Fernandez models.