Bumblebee goby:
They prefer brackish, I add only about 1 teasp of marine salt per gallon to the 5 that I keep mine in. They aren't particularly fussy about the salinity being especially stable, since where they live it fluctuates all the time, so I don't worry about getting it "exactly" to a specific gravity.
They are fussy about eating though and much prefer live food....I feed mine live blackworms, a pretty easy meal if you can find a place that keeps them on hand. I keep them in a "CLOSED" container in the fridge and change the water a couple of times a week. Toss a few blackworms in there and they gobble em up. A VERY cute little fish that I'm sure your daughter would enjoy, but a bit more work to care for than endlers or neons would be....because of the BW and feeding issues.
A couple of lousy pics of some male endlers that I have, these guys never sit still !
Endlers would be very easy to care for. You could have a nice group of these guys in a 5, with a few females as well. No special requirements and will eat most anything you offer them. You'd have to have someplace to put all the babies these guys will produce...lol...or put another fish in there with them that will eat them up....maybe one dwarf gourami or a betta. They are prolific breeders.
I'm guessing your daughter would like something with a bit of personality in there that she can interact with......something that will come right up front of the tank when it sees her..and beg for goodies. Bettas are really good for that and you could have a small school of endlers in there with it, depending on the personalilty of the betta. I had an older betta in with some endlers and he generally left them alone, but he did do a good job of cleaning out any fry they produced !

I think the older a betta gets, the more mellow they become. A young cocky full of himself betta might possibly clean out your endlers, but you could give it a try.