That’s a real tough one to answer. It’s bad enough the other way round as my wife and I found out, ( London to Aus/Nz) I think I would tend to concentrate on getting as close as possible to mainland Europe, and then maybe looking at paying one of the many low cost airlines now operating to get into Spain.Once there, the train is a good option. Good luck !
your chances of getting 5 seats together for all flights as rewards tickets is very slim. June/July is peak season and most rewards seats are now already gone.
but generally 5 seats would be hard by , unless you’re willing to have your wife and 1 or 2 kids travel on separate flights even on different days.
I would have to agree with Mark, particularly in premium class. However, you might have better luck in economy. Split party travelling is quite common with point hackers. Keith had to do this at least a few times when travelling with his 2 kids.
For options with using your different points, I like to use awardace.com to give an idea of how much it cost in different point/mile programs for getting from A to B. *Beware there are some errors. Double check with the relevant FF programs.
Another site I personally like to use is routehappy.com to get a quick comparison of quality for different flight options getting from A to B. They use a 10 points scale.
These are just my personal tools. There may be better ways out there.
Good luck. The kids would love the trip. I know I would :).