Too much information is missing. What's a tantrum? Was he loud, kicking and screaming or refused to remain seated?
As for changing seats US airways are a bit unclear as to that on their website even, so I'd complain about that. Who booked the seat is therefor not that important since you can't know that it won't be allowed.
Also they told you you could fly with a flight in 6 hours (presumably their next flight), declined, left the airport and arrived next day trying to book a ticket for the same day? Yeah, that'll cost you. Also if they offered you a flight in 6 hours they did try to accommodate you, travelling with kids I may have preferred not to spend 6 hours in an airport, either, but I don't think US airways will pay for that.