A Betta! They're hardy, low maintenance... beautiful and fun to watch. My 5 and 3 year old love their Betta. We keep it in a 5 gallon aquarium by itself. The filter is great, no heater is survivable although they do like warmer water.

The thing about bettas, in a single tank, you can only have one. Goldfish or guppies if you want plural.

white clouds would be a good choice. they are quite beautiful when young as their colors' intensity tend to fade somewhat as they age but they are very common, time-tested and relatively cheap for aquarium fish. i would put about 6-7 of those with a few olive nerite snails for algae control and perhaps a small corydoras (scavenger or bottom feeder) or two along with some low-light aquatic plants in a 10-gal.

white cloud mountain minnow

White clouds or zebra danios.

Goldfish are too messy and get too large for a ten gallon.