I have a six year old and between my husband & us we have tanks at the moment. My son & I like the same things you do, it seems!
We like our 55 with fancy goldfish the best! They move slow and their fins are beautiful - catching our eye - but they go all over the tank too.
They genuinely like each other and us. They do some funny things like pick up sand and spit it out.
If you have any questions, lots of people on here keep these too.
I also have & enjoy red serpae tetra (long-finned), Bolivian ram, platys, cory catfish, albino bristlenose pleco (long fin).
If you like any of these, I can give you more info on what they are like!