We took the older two to Alton Towers for the weekend, stayed in the hotel, went to the waterpark etc, for their 12th and 10th birthdays. It was brilliant. The staff were polite and helpful, the rides were great, we had a wonderful time.
We took the younger two to Disneyland Paris for their 5th birthdays and it was lovely because the magic was still real for them.
We took our 3 to Alton Towers for the weekend. stayed on site and had one day in the water park and 2 on the rides. However as bluecarrot says ask which they would sooner go back to and its disneyland everytime i am afraid.