I've done two breaks to Brittany with the kids (now 2 and 5). First holiday we went Dover to Calais then drove via Normandy and were staying not far from the north coast of Brittany. I've found it a very accessible holiday as someone with moderate French, easy driving, good food for adults and kids and lovely beaches.
The second time we went on the overnight ferry from Portsmouth to St Malo which was excellent. We then drove down to near the South Coast of Brittany and had a lovely time. Lots of beautiful towns, nice beaches (perhaps not as nice as north coast, but I think better, more reliable weather overall), and castles, etc.
This year we're doing the Vendee, via the overnight ferry again. So let's see! I've heard it's a far nicer, warmer climate in the Vendee. There's also the 'Green Venice' area which is perfect for boating and picnics.
I've previously spent a lot of time down in the Dordogne and Lot regions and if you're happy to fly then that really opens it up. The area along the Dordogne, east of Bergerac, is simply beautiful. Lots of opportunities for canoeing, walking, castles, amazing fortified towns, superb weather. Then there's the caves for trying wine, St Emilion, etc.
If I can be any more help, then shout!