Sorry, I don't have children so can't give you a parent's perspective on this - I can however give perspective from view of someone who moved to a foreign (non-English speaking) environment at that age:
If you are not planning to STAY in Switzerland, it is easier for the kids to go to International schools but to join activities for children in your neighborhood. The International systems are set up with curriculum that transfers more easily from country to country, usually probably with the thought you'd be moving to another area where going to int'l school will be more or less necessary, so continuing on there. Their curriculum usually fits relatively smoothly between US / UK type school systems from what I understand, so going "back home" will be less of an issue too.
However, easier isn't necessarily better .
Better depends on how versatile you and your family are, how fluid your ideas are about "formal" education vs learning things and life experience. Even going from US to Japan (to DOD school) and back to US, I and my siblings, as well as our parents, had to readjust and rethink grade progression. Not every school even within the US has the same criteria when you get to later grades, so dropping (or only loosely holding) expectation of "must finish school in X years" may help you put your head around easier vs better dilemma.
Is it better in your mind to go with opportunity presented for the children to be completely immersed in Swiss environment, or better to be able to move smoothly in and out of schools for the sake of school? To me, either has advantages and disadvantages, but only you can answer that for your family.
For me, for my experience, going from California to Oregon for just a semester, my teacher (2nd grade, in Oregon) recommended I be held back a year, partly because they were leaps ahead of where I was in Cali. Then we moved to Japan and back into a DoD school - and I was bored doing 2nd grade over again.
The next move with impact for our family was Texas, where my sis didn't fit in so well (Cali to Tx is quite a cultural adjustment) and she was "sick" a lot from school. She had enough absences that they recommended she stay back as well.
Next was to Hawaii, and back into a heavily DoD influenced school again with a lot of other kids "just like us" (military and otherwise parental-job transience) and sis did much better and my adaptable younger brother and I were in our element too.
Final move with school impact potential for me was a mid-year move from Hawaii to the Chicago area - and the curriculum was so different compared to my school in Hawaii, I'd have had to repeat my senior year ( ) in order to graduate from the Illinois system. I stayed with some friends in Hawaii and graduated there, while my parents moved on with my younger siblings.
So, even moving within the similar type educational system is no guarantee that easier even will happen the way we imagine.
Maybe taking advantage of "once in a lifetime" opportunity for your kids to go to school with the German-speaking kids from your neighborhood is better , but maybe keeping education as smooth a transition as possible (knowing there will likely still be some bumps!) is better for your family.