some of the "betta ______" things I've seen look so horrible... There was one that had a little opaque divider you could lift so they'd flare at each other across the glass... It was no bigger than a shoebox, and had a little valve at the bottom for "easy cleaning." No heater, no filter, and no room to swim. Might as well have been betta state penn.
Yes, bettas can survive in amazing conditions, but if you want to take the humane route when getting a betta- Get at least 2g per betta. If you'd like to get 2, get a 10 gallon kit ($25) from wal-mart, two halogen bulbs ($4 each- they look SO much better than the incandescent), a heater ($10) and a tank divider (easily DIY for free, or $2). Your bettas will have 5 gallons each to themselves, have much less stressful, healthier, longer lives (and when it comes to kids and fish, long lives = happier for them and you), and somebody else can even get her some corydoras or something to keep her bettas company. It may be a bit out of your price range- but perhaps multiple people can chip in for it? It truly would be the better way to go. Bigger tanks are easier to maintain, so if they're going to be taking care of these herself, 10g might be easier for them to take care of than a 5g.
If you still want to go with the smaller tanks, one betta would probably be better- I have a friend who kept two bettas in a 2.5 and they found a way around the divider and shredded each other... Not something you want... Not trying to scare you- just it's better to know what could happen in advance rather than to traumatize fish and kids.
I'm sure your sisters will love the bettas. They're amazing fish. Good luck with everything!