Instead of tank with fish, you could do something like this;
It's a 10gal walmart tank, with a 96watt PC fixture from Home Depot on it, and a cheap walmart HOB filter pad.
My sort of thought with your daughter, is that she can get her hands into the tank, pruning some macro algae out, maybe fragging a coral or two and watching them grow. Add some hermit crabs and a few snails for activity. The macro-algae helps balance the levels out in the tank, and with no fish, you don't have to worry about possibly transferring diseases in between tanks, if you ever wanted to add the corals to your main tank. In addition to a few different types of macroalgae, get some hardy, fast-growing, easy-to-frag-corals like zoas and xenia.
As she grows and ages, she can become more involved with the tank, doing water changes herself, etc. Have a gallon of premade-SW ready for her to go, and do it sort of as a responsibility thing.