Can anyone recommend any good drills for Girls U-12 to improve transition?

Players are having difficulty switching between offense - defense.

depending on where it happens on the pitch i would say, the first principle is everyone gets behind the ball and closest must press to buy a bit of time. i would do some practices that involve getting the team back behind the ball in the organisation you want as quickly as possibly. Closest pressing and how everyone reacts to that afterwards. I would maybe work with 2v1 against, 3v2 against and 4v3 against in areas of the pitch to start off with - because generally when you lose posession the other team should have an advantage of some sort - at least one player.