South Africa? Beach initially, might be whale watching season, Ostrich farm, hiking in the Drakensberg mountains, safari which is really special if you can splash out a bit. No jet lag too. It’s winter time but still 20 + degrees and sunny. The ‘Garden route’ is a good introductory itinerary.
Or Florida- yes Disneyland is a bit tacky but undeniably great fun for this age (and , suprisingly, adults too). Animal Kingdom is great. We liked Universal too with the Harry Potter stuff., two great water parks and we travelled around Florida visiting the swamps, alligator reserves, swam with manatees, watched huge rays in the water, visited the Space Kennedy Centre and Miami is fun. The beaches are beautiful in places like Anna Maria Island and kids love collecting the amazing shells. This can be combined with a Caribbean or Mexican trip but is enough on its own really. Lots of accommodation choices in all price ranges. It will be hot and humid and it’s hurricane season but there might not be any although you could expect heavy showers then the sun comes out. If you have 6 weeks, you can really slow the pace down as there is so much to see and do.
We also enjoyed a stint with dc in Jordan - combined the Dead Sea, Petra, Amman, Jerash and the Red Sea coast for beach time. Will be hot though. Maybe 6 weeks might be too much.
Also winter time in Australia but still warm on the Gold Coast and Darwin. You could stop off in Malaysia or Singapore.
Japan seems to be popular too, I’m hoping to go soon. I know lots of people who have been - kids love all the city lights and ‘strange’ shops, robot cafes, bullet trains and Manga stuff. Safe and easy to travel around. Will also be humid.
Iceland, Norway and the Scandi countries - amazing Fjords, boat trips, geysers and volcanic landscapes, thermal outdoor pools, midnight sun. There are some lovely beaches in Denmark. Should be fair to good weather although Norway is unpredictable. Not too far away and most people speak great English so easy travelling.