For fish. Since its for your daughter I'm guess you want colorful and flashy.
Platys, guppies, corydoras, Kuhli loaches, neon tetras... Etc..
All easy to take care of and active and/or colorful.
Many people don't like loaches but they are some of the funniest little guys you can own and Kuhli are great starter ones.
Maybe one day you could look at bigger ones.
My 7 inch adult dojo/weather loach:
Also!! Do sand. I'm biased to sand. Gravel is just so hard to keep clean even with vacuuming. Fish also like sand Better in most cases because it's smooth to the touch rather than sharp gravel.