I'd also second England - especially the South-West. Glastonbury is a fun (lots of new age stuff and historical sites - and there's much more around there (Stonehendge, Longleat, Brideshead), etc.

Also, in late June and most of July there's a ton of festivals (like the Glastonbury one) - many of which now cater to families with children (some only cater to families actually) - worth looking into. Also village and farm fairs with animals, steamrollers, etc.

Also a big hit with kids is a farm stay if you can find a good one - there are often cottages on working farms for rent. Kids will love being on a real farm - some with cows, pigs, etc. You'd have to be careful as farms can be dangerous - but the kids will remember for many years to come.

To be frank - my kids have traveled the globe, but if asked they insist they first return to SW England. Also, I am biased as I went there as a kid myself and felt the same way.