I came to Switzerland when I was 15, and went straight to public school. They have a transitional program for non-native speakers, which I had to do for 1 year to learn French, but considering the age of your kids, they probably won't even need 6 months. Put them in public school.

As for the gifted and talented programs, once they speak the language well enough, I'm sure you can see if they're ready with the appropriate people. I mean, I was able to get into "collège", basically their version of a high school that leads to university, after only being here for a year. If it wasn't too late for me, I doubt it'll be too late for them.

Maybe international schools are different where you're planning to go, but I knew people in Geneva who still speak French like crap even after living here for 10 years, all because they go to international school and never bothered to integrate. Plus there's the obvious cost.