Do you have a lot of these hanging around from the winter season?
You know, worn out, well-loved kid leggings, complete with holes in the knees and stains. Don't throw those out, especially if your daughter loves to wear sundresses as much as mine. I like my daughter to play without the worry of flashing her undies every time she jumps, kicks, sits down, swings etc. so this is how I turn worn out leggings into cartwheel shorts.
1.) Fold leggings together lining up the legs and knees.
2.) Measure, or guesstimate how long you want your cartwheel shorts to be and cut.
3.) You are done, you now have cartwheel shorts to go under sundresses. The raw edges won't unravel because knits don't unravel but if you don't like the raw edges you can turn them under and stitch it down with a twin needle or a long stitch. I don't mind the raw edges since they are going to be under a dress anyway. And there you go, no more undie flashing worries whilst playing!