Let me propose a slight variation to you, just going on what I get a sense that you're looking for overall (memories.) Hold off on the summer time frame, and wait until December. Then take them to a couple places in the UK (London, Manchester, etc.), and maybe Paris (if you're alright with that) to experience all the Christmas markets along with the other places. It really is a great time of the year to visit, and for several reasons. First, in the summer it is going to be crowded as all fuck no matter where you go. While there will still be people for Christmas, it won't be near the crowds that the summer brings. So things like the Louve, Tower of London, the British Museum, or wherever will be much easier to access and get around than in the summer. Manchester has a great Christmas market, London's big one is at Hyde Park (and more State Fair than Country swap meet and fair of the smaller towns), Paris was over north of the Louvre last time I was there. Brussels also has a really fun Christmas market as well, and isn't too terrible for not speaking the language. If you learn the local greetings/customs, as well as ask if they speak english, in their local language, people are generally more inclined to be polite since you're trying. Take 2-3 weeks, depending on things you decide you want to see, and work it out to try and get home the 21/22 of December. That's just enough time to rest from the trip before Christmas, but at the same time that glow from the trip and the memories made will still be hanging over the family Christmas day and bring it all together.