My first recommendation is to invest in a good guidebook; it's a tiny fraction of what you'll save during the trip by making wise choices and prioritizing the destinations and experiences that are most of interest to you -- if not RS, then perhaps Lonely Planet, France Eyewitness, or Michelin Green. You can also Google "France travel with kids" to bring up some websites full of advice specifically geared to families like yours.
With 3 weeks, you can really get a feel for France. Paris 4-5 days, then Mont St Michel and surrounding Brittany for 4-5 days ... from there you might want to head for the Bordeaux region, then the western (hence less touristed) end of the Loire Valley.
In Saumur, the Cadre Noir performing horses may be of interest, if they have a show on a date when you can be there (see http://www.ifce.fr/en/cadre-noir/visits-galas/public-performances/ ).
The Fortress of Chinon is a Loire historic site that's very kid-friendly, with sound effects and knights templar life-size cutouts on display.
Just upriver from the Chateau of Chenonceaux, you can go on a boat ride with La Belandre that takes you under the arches of the chateau's famous bridge. http://www.amboise-valdeloire.co.uk/prepare-for-your-holiday/visit/130176-chenonceaux-en-bateau---la-belandre