Children's Place and Old Navy both make skirts with built-in shorts (probably only up to a certain age, but I THINK 7 year old clothes still have them). These are the best! My daughters (3 and 5) live in them. Leggings under skirts or dresses in the winter. Can use bike shorts under skirts or dresses in the summer.

Tummy cover is a bit harder - maybe just tighter t-shirts and tank tops?

Get some monkeybarbuddies shorts for under dresses. Or get her some cute leotards (gymnastics gyms usually have a nice selection) and she can wear those with skorts. FWIW, I was the kid who was always cartwheeling, and I wore leotards a lot, with other clothes (because I didn't want underwear or anything else exposed).