Alligator Bay on the road to Mont St Michel is fantastic for kids. We were at Des Ormes last summer with our kids who were then 6 and 3 and they loved it. We went expecting to be in and out in an hour but it was a good half day.
The aquarium at St Malo is worth a visit. There is a round room you can go into which has a huge tank surrounding it full of sharks and giant turtles. There is also a submarine ride.
The old town in St Malo is worth a visit to walk round the walls. As you go into St Malo follow the signs for Navette Bus and they'll take you to a car park where you can get a park and ride bus to the edge of the old town.

We had appalling weather at Des Ormes (June/July 2007) but we still found the site to be very good. The carry out pizzas were fantastic. We didn't eat in the restaurant but it seemed to be very busy. The shop (as usual) is more expensive than the supermarkets in Dol but had a good range.

Perhaps it's a bit further than you were planning on doing, but there are all the D Day landing sites, Arromanches, Caen and the Bayeux Tapestry - which has a fabulous children's audio tour. And don't forget to try local cider, Calvados and Pommeau!!