So I was at Zion as they were shutting down in March. The first day the shuttle busses were running. The second day they weren't. The visitor center was shut down completely the whole time. The Lodge was open (we were staying there) but shut down right after we left. The horse stables were open too.
All that said, it's Zion. It's mostly hiking anyways. The trails should all be open (except those closed for maintenance). The only concern is parking, which is why the e-bikes are the way to go, especially for kids. It gives a small boost so you don't have to pedal hard for 16 miles. My 11 year old did it just fine. That said, I think there is a 12 year old age minimum so you may have to stretch the truth a bit.
also, they give you locks so you can secure the bike at the trailheads.
Also, if your girls like rock climbing or canyoneering, that is located just off the park boundaries so it's something awesome to do while not in the main canyon itself.