What do your kids like doing? Europe in the summertime tends to be expensive, more so with two teenagers. For reference, gasoline in central Europe is (ballpark) $8 (US) per gallon.

How about a family trip down to Argentina? Seeing Iguazu would be really memorable for your kids, plus as it will be winter down there, you could plan a ski trip either in the deep south or at any of the Andes resorts. Beunos Aires is a nice town for tourism as well. Things are quite a bit less expensive down there.