Being a 'fat-tired' family (MTBs only), I'm in a similar position. Daughter just turned 13, and while I don't think she's gonna grow as much as yours, I don't think she's done, either.
She has an 'upgraded' BBS bike right now, and I believe it will serve her until I know better where she'll end up. Then, she'll get the Sette Ace she wants.