First: Welcome to AA!
Second: Do you know about cycling? This process, which can last a number of weeks, will make your tank safe for fish. Otherwise, dangerous chemicals will build up in your tank.
Fishless Cycling For Dummies
Finally: Once your tank is cycled, I think a Betta fish would be a good fit for your tank and your daughter's wishes. They are pretty, fit well in a 5 g, and have distinct personalities.
I am relatively new myself (only one month in). I am sure much more experienced members will offer better advice, but I wanted to make sure you got this message before you went to Petsmart. I wish I had known of this site before I started out! I highly highly recommend cycling the tank first.
Once cycled, Petsmart should have a wide selection of Bettas in different colors. I think the basic varieties of Bettas cost under $5.