Ooooooooohhhh you two - you sound so sensible! I love it!!!
The most important thing when buying fish for children (believe me, I have learned) is to try very hard not to buy fish that will DIE - either because they are "fragile" or because they will eat each other. Not like monster fish, but in a nippy way. Kids hate to see fish floating because their tails got chomped off.
Rule #1: it's either all barbs or no barbs (see the tail chomping part)
Rule #2: Avoid "fancy" fish (goldfish and guppies) and in general stick to fish that are hard to tell apart from one another (you'll figure out why when the first one dies).
Rule #3: Kids don't ever want to return or give away a fish (why I had those darn giant danios for so long) - so NO PLECO - EVER!!!
Dwarf Gouramis would probably work...
I like the idea of the tetras - I would get all the same kind because they prefer to school. If you don't like neons or head & tail-light, you might consider skirt tetras or rummy-nose tetras, about 6 of any one species. Please don't overcrowd and remember that if you are gifting a tank to a child, you will probably also end up maintaining it. Bonus - they can be fed just flakes.
It's a very nice idea. My 9-year old is quite a fishkeeper. Beware of letting the child make the choice unless you are prepared to say "no" a LOT and deal with some disappointment.