I've driven through France three times, and driven to and from Normandy, Brittany & Haute-Veinne (in the middle) but always crossed by ferry, evenly split between overnighters and Dover-Calais. Paying the motorway tolls is worth it (unless you've got the time and you're planning to take a leisurely drive), the motorway network is very good. If you're going to Paris, Normandy is accesible and not outrageously far away. Rouen is worth a visit, iirc it was the home of the Cluniac monks who founded Paisley Abbey and Rouen Cathedral and the Abbey share some structural similarities. Not too interesting for a 4 year old, mind, but the town centre's well worth a wander.
When we went to Normandy the first time we visited Wiliam the Conqueror's castle in Caen and my son (aged 11 at the time) spent a few hours outdoors in the castle with a supervised group, none of whom spoke English (although the supervisors did). He built a model car from all sorts of rubbish - he loved it, and it gave us time to have a wander.
Apart from the castle, there's not too much to see in Caen itself, it was heavily damaged in WWII and rebuilt, but you're not far from some important WWII sites, and not too far from Bayeux if you fancy that. Might be a bit too far from Paris for you, though.