Doesn't ITA work out of US inventory, and thus price things like aa.com rather than ba.com?
Back to the OP, I am no expert on kid's activities (though the other maintains I normally behave like one) but any of the Central/Eastern European cities are fun to visit and good to visit. Riga is lovely, VNO too, KRK charming and PRG magical if swarming with people from Podunk, Illinois.
If you wanted more beach, Malta would be a great option with two decent Starwood properties....but it isn't on the list.