Hi, welcome. Let's see. I'm on the other side of the country, not Lausanne, so can't speak to the particulars. But as far as stimulating your kids, here are some thoughts based on our overseas experiences first at ages 5 and 9 and then at 12 and 16:
1) Consider giving them a head-start in French before they arrive (Michel Thomas, maybe? I haven't used them but Rosetta Stone wasn't analytical enough for my kids, though it did get them started.) Having enough to have a basic idea of what's going on makes a huge difference in making friends. Concordia Language Villages does very well-regarded immersion camps for kids.
2) Consider taking on-line (breadth and depth, not acceleration) classes through artofproblemsolving.com, if they have the time and interest.
3) My impression that in most schools here the curriculum isn't very flexible, and the main accommodation that's made is acceleration (skipping grades). Things do speed up quite a bit in gymnasium compared to primary school, though.
4) Music has been a great way to integrate -- there's a limited vocab and a lot of clues about what is trying to be conveyed, as well as an instant community, esp. with chamber music. You're right, music is one of the routes in.
5) I totally second the idea of a Chinese nanny -- if you can find one -- it is absolutely the best way to support a truly foreign language, i.e. that isn't the language of the home or the community. It worked wonders for us, not to mention the friendship that develops. But even if you drop the Mandarin for the time being, the foundation of being able to hear and produce the tones will still be there when he/she gets back to it.
6) Most importantly, I also second the idea of seeing this as an ideal learning opportunity, for two reasons:
6a) What better way to learn a language? learning German(French for you) for "free" (in the sense of naturally, not being something tacked on) was one of our motivations to come here, one of the gifts that this time brings us. I see that you are weighing the pros and cons of this, and yes, they'll need support for the stress of it, but it's worth it. In some ways, adding languages gets easier the more you do, and before puberty is the time for it, to end up with a native-like accent.
6b) I think you may already be clued in on this, but the experience of things being difficult is one of the most important experiences for profoundly gifted kids to have, and one you should seek out if they haven't had it. Check out this link [ [http://www.nymag.com/news/features/27840/index.html]](http://www.nymag.com/news/features/27840/index.html]) or Google "How not to talk to your kids: the inverse power of praise" or PM me for other articles.
7) I assume you know about the Davidson Young Scholars program, for support?
8) In Kanton Zürich, there is a school called Talenta ( www.talenta.org ). Maybe they can give you recommendations for Lausanne.
Sorry, I was writing in parallel and I see that some of what I have written above is redundant or irrelevant.