I've got two 5.5s, one with a betta and a pair of ADFs (my six year old's tank), and one with 3 dwarf puffers (my 8 year old's).
You can go with 5 or 6 of any of the small tetras or rasboras - neons, green fire, ember, black neon, and von rio tetras, any of the bororas rasboras - brigittae and maculata spring to mind. Medaka ricefish. Certain killifish will work. Some of them will eat prepared foods, some not. Ask a killi expert.
Sparkling gouramis, Honey Dwarf gouramis (Colisa chuna).
The smallest cories will work, but not as well as you'd think, because they all prefer to be in as large a school as you can manage, generally 10 or more in the aquarium (thought they will survive just fine, you might not see them often.)
Some of the smallest Lake Tanganyika shell dwellers can be kept as a pair in a 5.5, but then you have to have something to do with the fry. Culling is no fun to explain to kids. Trust me on this.