The case saver is almost a must have. The bushing helps keep things tighter on the inside as far as bearing slop. Even with the case saver and bushing, if the kid lands in the gas, the gears will break. If you are having issues with repeat gear breakage, I recommend you put on a slipper sprocket. The slipper has issues of it's own but it's better than busting gears, and even better than what's next - busted cases.
WPSA 90 CVT #2
07' APEX 90MXR
Probably not the fastest, but hopefully fast enough...