I have in the past maintained a 5 gallon tank with 2 mollies and 2 platties. We ran into trouble only after a move - a fish got stressed and jumped out, a new fish was bought and not quaratined and... well you all know that story. I tested my water quality regularly and the tank was in great shape for a year before the disaster, and it was my first ever tank. Super easy fish. I also think you could get away with a school of 5-6 of the tiny tetras (e.g. neon) in there.
my kid would want danios but... that's minimum 10 gallon long tank for a decent school. they're super fast zippy silvery fish that are active and fun for kids to watch!