[PHXFlyer]

If I may make a suggestion Continental serves both Ontario and Austin with a connection in Houston. Checking some random dates over the next few months their fares were almost always less than US Airways by as much as $60 per person for a round-trip ticket. I believe the unaccompanied minor fee is the same. Continental has a dedicated lounge for unaccompanied minors where they escort the kids during layovers or long delays. There's TV, games, books, refreshments and their own restroom and it's only for minors traveling alone. There is always an escort available who will take them to buy snacks or meals at an airport concession. If your kids don't travel with a cellphone they also allow phone calls and will give you the direct number to the UM lounge to contact with any questions or concerns.

I also know for a fact that Continental will put them up in a hotel, supervised at all times, if their connecting flight(s) are cancelled even due to weather. The last time I was stranded at IAH there were UMs who missed the same flight as I did. They took us to the same hotel where they house employees who are in Houston for training. (Continental may even own that hotel but I'm not 100% sure on that fact.)