Hi Stooo,
You may want to look into the Swiss Alpine Club
www.sac-cas.ch - they probably have a section in Geneva where they may even have a Kids Mountaineering Group as they do in my section. Our kids get together once a week for climbing at a gym and often go on weekends and camps together. If they are not so into the vertical world, I find that the local afternoon athletics that most towns offer for particular age groups are great for integrating kids in a fun environment (assuming the leader has a good handle and thread to the kids). Ask the town for info on athletic clubs. In addition, have a look at the Home School Association of Switzerland - here we run many events and connect members with similar interests (may be you would like to join?). And here the link:
www.homeschool.ch/en_index.html
Hope that helps,
Cheers,
Olivier