DH has traveled a fair amount for work, for several years,
Getting the kids' passports if you don't have them already will cost more than that! Wow! That's so cool.
I'm looking for a good deal on somewhere in Europe too. I've decided to fly to Detoit to see my family for my niece's high school graduation, and then I like to go on a vacation from there. So I'm thinking London since we haven't been to Europe and I'm a little intimidated by the language barrier anywhere else.
I'm thinking of doing a home exchange though to really keep costs down. Have you considered that?
Fares to London are very low this time of year, and also January through April, because the weather is unpredicable (i.e. pack a raincoat and warm sweaters).