Wal-Mart has an in-store only 10 gallon set up for around $30. You'd need a heater (get an adjustable one). You, your daughter and neices could build a divider and put both Bettas in the same tank (unless neices need separate tanks).
If I were to buy another five gallon, I'd avoid the Wal-Mart Hawkeye with LED lights and get this one:
Acrylic so you don't have to worry about glass breaking; lighter than glass for sitting on bedside tables and plenty of room. They could even add another fish or two later on after they get the hang of caring for their Bettas. Even a couple of African Dwarf Frogs.
Five gallons are a lot easier to take care of than smaller tanks and require fewer water changes. Plus, with this one's hood they can have live plants if they wish. Dimensions are 16.5 x 9.38 x 13.75 high. Is that too big?
Welcome to the forum!