if it does well for you, survives and thrives, it will outgrow your tank pretty quickly (less than a year, IMO). while you may or may not get attached to it, your daughter certainly will. my 5 and 10 year olds are pushing me to set up more tanks for them because they can't bear to get rid of their multiplying guppies. my suggestion that I feed them to my leaf fish and seahorses were met with total rejection. I don't have much of a leg to stand on against tank expansion, since they see me doing it (look at my sig). we set up more/bigger tanks for guppies this weekend... sigh...
I agree with the previous posts that it is not unethical to buy a fish you know will outgrow its tank IF you have a plan to get it to a suitable home (via petstore or direct to a hobbyist). but since you are not planning to upgrade to a much larger tank, be aware your parental ethics may be questioned by your child. kids are not understanding if for any reason they have to give up a pet.
I'd find a few engaging, colorful, small and hardy fish that she could enjoy for many years. you didn't ask for a list, but I'll give you a few that IME are hardy and the right size for the tank. these are not all necessarily compatible, just suggestions for your consideration. also, I'm not telling you not to get the tang, just warning you of the potential consequences at home. if you do get one, don't get the teeniest ones. they can be really delicate and often don't survive. sometimes even the difference between a one inch tang and a 1.5 or 2 inch can mean survival in your tank or not. go for one really filled out, active, brilliantly colored, with no transparent or faded areas and no white spots anywhere.
some suggestions for small, colorful, hardy fish I have kept for at least three years:
-six line wrasse
-royal gramma - I had one of these for over seven years
-pair tank raised ocellaris clownfish - you will end up with a bigger and smaller one - Nemo and Marlin to your daughter
-orchid dottyback
-bicolor blenny
-coral beauty dwarf angelfish
a great reference for you and your daughter, even though you have hobby experience, would be New Marine Aquarium by Mike Paletta. great illustrations, a nice way to discuss fish selection with your little aquarium hobbyist. I have shared my hobby with my daughters since they were very young. it is really nice to have this with them. I taught my older daughter how to acclimate fish this morning.
I lent out my copy of the book, never got it back. I'm going to buy another one, even though I've kept marine aquariums 30 years.
to help with nano tank and beginner fish selections, liveaquaria has some nice suggestions:
nano
http://www.liveaquaria.com/product/scateg.cfm?pCatId=2124
beginner fish (some will be too big for your tank, many are suitable)
http://www.liveaquaria.com/product/scateg.cfm?pCatId=1926
Good luck!