Yes, schools in Zurich will be in (Swiss)German, although there may be other foreigners in your kids' classes since there are plenty of us! Have you started German lessons for the children already? If not, do it asap, just the basic stuff like where is the toilet, please thank you and so on. Having some basics will give them confidence although I'm sure the younger ones will pick it up in next to no time. (I was expatriated at age 7 and it did help to know I could say a couple of things).
For your eldest, this is pretty much a guess but hopefully will spur someone on to contradict me

: I rather think having a personal interpreter in school is unlikely. It's more likely that if they think he can't cope they would hold him back a year or more (our neighbour's son, who just moved here, is 14 and currently in school with the 11-year olds due to lack of German). There may be special German-for-foreigners lessons at school too, certainly our school does them (free) but we're not in your canton and it varies a lot. We also have teachers who work with special needs kids for a couple of hours each week to help them catch up if necessary. The name for them is sozialheilpedagog(in), I think, so you could see if there is one of those at your school of choice - again, don't know how it works in Zurich.
If you search for "autism" or "learning difficulties" in the search box at the top right, you should come up with some helpful threads - which may be why no one has answered yet.
Sorry I can't be more helpful.