Being there in early April won't complicate anything. You have a greater chance of some rainy days that early in the spring, but you could have fine weather then and rain in May or June for that matter -- you can't build your whole plan around possible weather, just bring appropriate clothing.
It's not clear how long your total trip is, but if time allows you might want to give yourselves a night or two in Paris when you arrive, perhaps at the hotel you liked so much before. With only 1 1/2 days in the city on your earlier trip, there would be plenty more to see and do. Then you could make your way to Gare St-Lazare and get that train to Rouen. Then rent the car for Normandy and the Loire. As Laura says, the Loire isn't a realistic day trip from Normandy, and it really justifies a few days itself. So whether you try to see both Normandy and Loire depends partly on how much total time you have. My own preference is not to move around too fast, spend at least several days in one place or another to fully appreciate it, but you may prefer a faster pace.
Most on this board advise against driving from CDG to anyplace very far (like Normandy) on the day you arrive after an overnight flight. Unless you sleep well on planes (which is rare), your jet lag and fatigue will likely affect your driving. The airport is surrounded by expressways which aren't especially challenging to an American who's used to our metropolitan areas, but it might be "a bridge too far" for most of us after a long flight with little sleep. However, returning the car to CDG at the end of your trip would not be so difficult, and would enable you to spend your final night at an airport hotel (we liked the Ibis but there are others) to be sure of making your flight home.
So one plan might be: Land at CDG, take RER or taxi or Air France bus to Paris hotel, couple of days there, train to Rouen, car around Normandy and Loire, drive back to CDG for overnight and flight home.
Another might be: Land at CDG, rent car and drive to Rouen, rest of Normandy, Loire, back to dropoff near Paris (Orly?), a couple of days in Paris, RER/taxi/bus to CDG and home.
Hope all that is some help.