Possibly doing the drill Balswick is showing? No stride
I think the Bosu would be great for balance, core strength and just plain fun for breaking up a routine. I'm not sure that alone it's a fix since it doesn't address the lower half mechanics for creating a good posture and torso/hip angle.
Yes.. no stride.. just to keep the girl from leaning and to develop proper muscle memory. They must stand tall and balanced on the Bosu when doing snap ups.. if they lean they will tip over. Also my DD stands on the ball side of the Bosu not the flat side. Makes it even harder to balance if she leans.