I would nix the brittle star because they are aggressive and they hide. Plus, they get quite large.
I would go with the chocolate chip starfish. They are very easy to take care of, they would be perfectly happy in a 40 gallon, and your daughter would get to see it quite a lot. I see my starfish everyday, sometimes more than I see my fish. My starfish actually comes to the top of the tank to eat, and my niece absolutely loves my chocolate chip. Plus, they are cheap. Most places sell them for like $8 so you could even get two if you wanted. Just make sure you give them standard acclimation of about one to two hours drip method, and that you feed it meaty frozen food (krill, prawn, etc.) at least once per week ( I feed mine every time I feed the fish, which is 3 times per week).