In reply to MattDTC:
I would suggest the Abondance or Aulps valleys just south of Geneva. Its not too long a journey from Calais, and has a good balance of child friendly mountains and Lake Geneva not far away for a 'beach'. The portes du Soleil offers a summer multipass which gives you access to tennis courts, swimming pools and chairlifts for walking if you want to use them for €6 a day. Theres a fair number of activities on offer around as well if there is a rainy day.
The Abondance side is a bit more French than the Aulps side, both are very friendly. And lots of campsites!
(I live here so give me a shout if you want to know more!)