Following up on my post from about 15 min. ago. Just got off the phone with Air France who said they are not renewing their contract with American Express to use rewards points for flights. I know I was not the only one here looking to use those to save on airfare. Have to try another airline now. Hopefully it won't cost more money.
If I had actually thought ahead beyond the initial cost of the cruise, I might have thought twice. We are using 900+ DVC Vacation points for one adult and one child. So the cruise came to $6973 for a Cat 5 room. However, add on airfare from DC to London, and then London to Barcelona, the excursions, and all the other items that you mentioned. Now the trip costs in the high teens or low $20Ks.
Bring plenty of stuff to occupy the kids on the long flights. Make their trip fun from the moment they get in the car to go to the airport. 

