With all due respect, I have to disagree on the 31/21 recommendation. That's way too much bat for the DD described in this thread. 30/20 would be the biggest I'd suggest but that depends on the bat type as well. i doubt she'd swing an Easton that is labeled 20oz very well but may do ok with a Demarini at 20oz.
I dont suggest weights as a ton of people have their own advice on here.
What I do however suggest when people are looking to move up, is to weigh your current bat for real weight. You never know what her current 30/19 metal Easton weighs.