Charlie (about 5 feet long) was in a huge 25 gallon tank and it was a nightmare to keep his humidity up. Penny (about 3 feet long) was in a 10 gallon and it was easy, but I was about to upgrade her to give her more room and a second hideaway.

I see nothing wrong with upgrading to give her more room as long as she's happy with her hideaways and the temps and humidity stay where they should, and would recommend getting something other than an aquarium to upgrade to if you do. There's some great options out there when it comes to tanks, and even though they might get expensive, it's certainly worth it. You can try a PVC tank that has a glass front so your daughter can still see her pet any time she wants without having to put it in a tub or rack. I was going to order one from PVC Cages myself. They look pretty great, and there's several companies online like them that make similar products. And there's several places that offer a wide range of hide sizes so you can upgrade that as well.