Congrats on the potential move up. I'm sure your daughter is very excited. I would ask the coach to be a little more specific as far as strength. Does she need to work on upper body more, abs, legs??? Is she getting some basic strength work in her classes now? See if the coach wouldn't mind spending a few minutes showing you some things she could do at home. That way, you and the coach are both working toward the same goal. The suggestions that were given here were great ones.
My daughter used to just get her bike out and go for a ride. Her older sister would ride along and the goal would be to ride continuously for a certain period of time---she would gradually do longer and longer rides. Its not as structured as straight exercises and got her outside. It does help build leg muscles and certainly cardio. If you have access to a pool, swimming is great! Its non impact, helps with cardio and she can work all the muscles in her body.
I think the challenge with the younger ones is to do the strength in a fun atmosphere otherwise they get bored/lose interest because at their age, they don't see how doing a bunch of v-ups will help them get that kip on bars.