I'd suggest a Singapore, Thailand, Hong Kong loop. Lots of hrs on the plane, but those societies like kids, have a large variety of international exposure and are each cheap.
South America might be good too. (but winter there in our summer!)
Try to avoid tourist stuff and experience what the 'locals' do. Stay with host families with kids. (very cheap and fun too) We used a Homeschool guest home directory, but others exist.
These larger cities (Singapore and Hong Kong) are very ez for language and they have nice Zoo's and authentic cultural areas. The zoo without noticeable fences / cages in Singapore was delightful. They didn't get to do 'night safari' there, but I think they would enjoy that too + the 'laser Light show' at Sentosa was a hit. Just eating at the local food courts in Singapore, Malaysia, Hong Kong, and Thailand was CHEAP entertainment enough for the day.
Another choice could be the ethnic areas of Vancouver BC.
Montreal or Quebec City is also fun. For Europe I would do Denmark and The Netherlands.
What do you like?, what do your kids like? What are your best experiences and goals for the travel?
We had our kids do planning and make reservations (assisted and approved, of course
), as well as run the budget and exchange money and buy goods in each country (that ends up being a real 'opener' for getting to know the locals. They (boys) loved the great military museums in foreign destinations. Public Transit was harrowing for our Red-head, everyone in Buddhist countries had to rub his hair (word would spread for car to car on subways), monks would come running towards him with outstretched arms! Pretty intimidating for a 6yr old!
Have fun, should be a blast