Thanks. Keep in mind that I just brain dumped a massive list of things I've done and would recommend. All the stuff I saw as a kid was over an 8 year period. The Asian portion except for Saudi, Singapore and Japan, were all done during the last decade that I was living as an American Expat in Australia from 2010-2019.
Six weeks, of course it really depends what you're into and also like I said, I made multiple trips to a lot of these places so I didn't see them everything all at once. Example - taking a train from Amsterdam to West Germany to Paris and back through Belgium was one trip. Going to Haarlem and seeing Windmills and Tulips and staying at the Amsterdam Hilton (like John and Yoko) was on a different trip and I think the Anne Frank House was on a third trip.
Here are the top things that I can say I did as a kid that really had an impact on me.
1 - growing up in Alaska taught me a healthy respect for nature, the environment and severe weather
2 - going to Kenya taught me how the animal kingdom works on land. From living in a fishing village in Alaska, I already knew how the sea / marine life part worked.
3 - ancient history - Athens, Rome, Cairo, Luxor, Valley of the Kings, Madain Saleh
4 - culture - Arabs in Saudi Arabia and Kuwait, Africans in Kenya, Egyptians in Ancient Egypt, Europeans in 30 year post WWII Europe, Asians in Singapore and Japan
5 - what people can achieve - The Pyramids at Giza, Luxor, Valley of the Kings, Heidelberg Castle, Westminster Abbey, The Eiffel Tower, the Coliseum, Acropolis. and of course my dad worked on the Trans Alaska Pipeline
I'm kind of brainstorming out loud here. I think I'd group it like this:
Culture - England, Germany, France, Italy, India, Spain, Japan
History - Egypt, Italy
Achievements of Man -United Arab Emirates, Japan, Shanghai
The Future - Beijing, Singapore
The Environment - Beijing
So after all of that, let me think about a sequence of events.
Week 1 - USA to England - get your fill of curry, Big Ben, Parliament, the Underground, etc.
Week 2 - Southern Germany / Bavaria - probably stay in Stuttgart with day trips out to Heidelberg, The Eagles Nest, Schloss (Castle) Neuschwanstein (the Disney castle), Nuremberg, Baden-Baden, Munich, etc.
Week 3 - Rome, Italy - there is just so much history to see in and around Rome. The Vatican is going to take a day by itself. The Coliseum will take most of a day. You've got the catacombs, Trevi Fountain, the Spanish Steps, Pantheon, the Forum, etc.
Week 4 - Egypt or Kenya - you could easily spend a week in either one.
Week 5 - Asia - this one is tricky - I'd say Japan is probably going to be the most rewarding. I feel like China should not be missed either. I think you could get away with spending 2-3 days of connecting flights through Singapore if that makes sense. Singapore is worth a look but, I don't think it's worth an entire week.
Week 6 - probably consumed with travel between these destinations and back to the USA.
That would be about 6-7 countries with 3 continents over a six week period.
Of course you could just bounce around to a new city every 3 days but at the end of that, you probably wouldn't remember it was Barcelona or Madrid, Munich or Frankfurt, Rome or Milan, etc.
The other wild card on Europe is adding a week to travel to a former Warsaw Pact country and see what life there is like today. Maybe Poland or Hungary would be a nice place to experience that.
Last thing I'll add is Europe and the Holocaust. For your kids, I think it's very important for them to see first hand either one of the concentration camps, or the Anne Frank House, etc. It's much more relatable once you've been there. Kind of like standing in the window in the 6th Floor of the School Book Depository in Dallas. Once you've stood where some major historical event took place, it does give you a little better insight into what could have happened, or how things might have happened. Kind of how I crack up every time I see the picture of Adolf Hitler in front of the Eiffel Tower. I've been there. I've stood in that spot.