think you'd be best doing a search, and looking at the web sites of a few - we stayed in this area a year or two ago, it's my favourite place! the road leading to mont st michel has dozens and dozens of campsites - all shapes and sizes. due to me not booking in advance and our car breaking down on the way - we just pulled in at the first one we saw - cos it was nearly midnight. it was a lovely little site, very basic, but had everything you need. the closer to the mount the busier tho.....
You might like to consider St Michel, ACSI site 400. We stayed there for an overnight stop in Sept 05. The welcome, pitches, facs were good. Heated pool was excellent, but I don't recall a childrens section, so you might want to check that with the website. There was a small area with animals like a mini farm. Walking distance of the village. It's not on the coast, but bear in mind that in this area the sea goes out a very long way, so a site with a pool is a good choice. Cost with ACSI card is 10€ for pitch, elec and 2 adults.
It depends on how close to Mont St Michel you want to be. We have stayed at Le Vieux Chene at Dol de Bretagne a couple of times. Its a pleasant family orientated site. June is a good time to visit Mont St Michel as it should be a lot less crowded than July/August and you should go and watch the sun go down from there. As for sea fishing there, it's like Morecambe Bay, the sea disappears for ages.
Don't forget to leave a review of the French and other European campsites you have visited!