Listen, I think sleeping in Giverny is fine as a practical, driving safety decision - it's just a lost night (on a very short vacation) in a place with limited tourist opportunities. You may get there at 12:30 or 1PM. (Note: This is not a slam of Giverny, we went to Giverny by train as a daytrip, on our very first trip to Paris, seven nights near the Opera Garnier. We loved it. But not to spend a whole night.)
I don't think you are willing to change your destination list. That's certainly your right and your decision. But you need to think in detail about the consequences of your travel details. It sounds smart to not return the car inside Paris. But think about the seat-changing public transit overhead of returning the car at CDG: Rental shuttle, RER or bus to Paris, Metro to hotel? And you were worried about changing train stations to get to Bayeux! If there are two or more kids, I'd consider a taxi from CDG to the hotel, especially on a weekend. (Lighter traffic.)
If you had been to Europe before, you might feel more confident about train usage. A familiar, American-sounding car rental plan is comforting. And in fact, a car is very useful for Normandy and MSM. You probably should rent the car. But you are losing some options by doing so. (There is a tiny chance you'll have to accept a standard transmission despite your reservation.)
Here's another idea to consider. I hate changing hotels. But if you want to take the kids to Paris Disney ( ... not my favorite place), would you consider returning the car at Disney, sleeping there one night, and going to Paris for the remaining nights? That way, the train from Paris Disney deposits you downtown, ready to discover the city, in a well-rested state. Maybe someone who has been to Paris Disney can tell us if it's like an American car destination. I haven't been there, on eight stops in Paris.