As far as I know, angel fish, certainly more than one, would grow too large live in a twenty gallon.
You can check fish profiles to see tank size recomendations.
I was thinking a dwarf gourami might be a good "large" fish for a twenty gallon, and could have lots of tank mates, but you cant have two males. do the kids each want their own fish?
Do learn about cycling the tank. it will save you a LOT of headaches.( not to mention what it will save the fish)
One thing I'd recomend is Cory fish. They dwell in the bottom of the tank and are pretty cool and sort of 'personable' fish. they are also reasonably hardy, IMO 'beginner fish'. they like to live in schools. Three would be a good number for that sized tank, I think, and leave you room for other fish. You can get them small and they grow ,which is fun to watch as well.
Common goldfish wouldnt thrive in a tank that size, tear up plants and require a lot of water changes.
Possibly two fancy GF, but it would be pushing it, and you couldnt have any other fish. also would require a lot of awater changes. IMO, stay away from the goldfish.
You have Lots of choices with a twenty gallon. I'd say look around and see what interests you and check back, like you did with the goldfish.