Hi, Normandy is a lovely place, we've just come back from 2 weeks there. I have 3 children, similar ages to yours and found plenty to do.
The beaches are beautiful and there are many to choose from. My DH really wanted to see all the landing beaches and war stuff. Amorranches (sp??) beach is nice and there are bunkers on the cliff top that you can go round for free, the dc loved exploring this.
We saw leafets for theme parks, zoos and all the usual stuff you would get in England but we didnt try any of it as we found plenty to do anyway.
On the coast road heading west directly from Caen there is karting for the younger children, pony trecking and you can also hire family bikes which seat between 4-8 people which we found really fun.
There are always markets on in different towns.
Mont St Michel is definitely worth a visit, but, it gets very busy so I would suggest going early.
To be honest, there were loads of places we would have liked to have visited and more things we wanted to do but the site we stayed on was so good that the dc were happy to just stay there and go to the beach.