I honestly don't think you could do 1.5 years in an international school and then move them, not at their age. Imagine them trying to make friends, all the time thinking 'why bother, I won't be here next Sept'. And you'd have the 'move to local German-speaking school' hanging over their heads all the time, getting bigger and scarier as it got closer.
The German they would pick up in a year at international school would be the same as they will pick up with a few weeks at the state school. Honestly, many of us here do understand your concerns (I put my 8 yr old into a local school in April) but it's astounding how they manage. And what's the worst that can happen? They'll not cope for some reason or another, or not make the cut for the grammar stream, and you can move them into an international school then.
Why not think of it in reverse to your original assumption - 'let's try the state school for 1.5 years, then move to international if necessary' (but obviously don't tell the kids that - my son doesn't even realise there IS an option involving being schooled in English, because I know he's a lazy little tyke!).
The schools here are totally geared up to non-local-language speaking kids, and there will be a support programme for them for as long as they need it. And the split between the grammar and other streams isn't a one-off deal, at least in my canton (Vaud). Here, they stream the kids into one of three tracks at 7th grade, then there's an option to move up a track at 8th grade if 7th went well; then after 9th grade kids can do an additional 1-year course to boost them up and give them a leaving qualification the same as the track above. See? Tons of options! (You can tell I've looked into this as I too was that fretting, 'it's so unfair if he gets downgraded for language' parent ).
Hope this helps with your decision. If in doubt, make the one that's easiest to undo, which in my opinion is Swiss-to-International, not the other way round!
kodokan
PS: My son isn't one of those 'fluent in 3 months' kids; he doesn't work very hard at it and isn't gifted with a natural ability like his 4 yr old sister, who has only been at school since Sept and is already chattering and singing to herself in French. But he's very happy - the other kids have been entirely welcoming and adopted him like a sort of pet, and he's slowly getting there. He understands a lot more than he lets on, I think, and is just starting to say the odd phrase in French when he's speaking to us, without him even realising he's doing it, which is very comic.
I'm taking a sort of modular view to his education; he doesn't need to study all subjects simultaneously for the whole period, as long as he ends up educated as an adult. So at the moment, he's very heavy on Modern Languages and doing almost no English Language Arts other than reading. Lots of crafts and gym, no English History. It'll all come together at the end, though... I think...
Note to Mod: perhaps this should be moved to Families in a day or two, once the OP has had a chance to come back here and find the replies