well i cant suggest a heater due to never having smaller than a 30 gallon tank but i can suggest a full ulta omega easy to use deluxe(sorry i wanted to use awesome words) full 10 gallon kit that would allow for a betta or a nice school of tetras or some fancy guppies.
.AQUEON KIT DELUXE 10GAL
its $47 bucks, i mean hell if i didnt already have 2 tanks and soon to be a 3rd that is 150 gallons i would have gotten this for the kitchen of my house with some nice blacklight tetras.
check it out, it maybe something to work towards so your kids can really enjoy it.