I dont think the 3 feet difference is going amount to anything.
I'd stay down at 12u and build her confidence at the plate AND work on pitching more. Maybe practice with the older team some or pick up with them some too. I am a big believer in playing is better for then girls than sitting on a winning team. I know there is some psychological benefits to learning how to win, but playing is very impoartant. IMO
Do not argue with an idiot. They will drag you down to their level and beat you with experience.