Rent a car at airport - or outside?

I would always warn against driving in Paris. But CDG is already well out of the city centre, with relatively easy connections to the major autoroute A1.

There is a train station at CDG but I don't believe it has any services to Normandy. You may have to go into Gare du Nord or Gare de l'Est and transfer over the Gare St Lazare for Caen or wherever you're going in Normandy.

Airports usually come with additional taxes/tolls vs. picking up cars in center city locations.I
But in the case of CDG, I would just suck it up and pick the car up at the airport.
After living in Atlanta, I will drive anywhere. But Paris traffic is just not any fun.

Thank you! We'll plan to pick up a car at the airport and head out!

I assume you are flying into Paris from the U.S. I strongly advise not to drive the day you arrive. You will be tired and jet-lagged. You will then be driving a strange car on strange roads. That's not a good combination. I did it once flying into Dublin and immediately hopping into a rental car with the added challenge of driving on the left. I was lucky and nothing happened, but it was still a bad decision. Fly into Paris and take the train to Rouen or La Havre. Rent your car there and start fresh the next morning.

If you are arriving from the U.S., then FastEddie makes a lot of sense about driving immediately after arrival. Just not a good idea for most people. Some can handle it, but others are somewhat impaired by jet lag, and others are complete zombies. Even a minor fender bender at the start of the trip will make for a less than ideal time the rest of the trip. Just food for thought.

If you are flying overnight, don't drive to Normandy on arrival; you may be willing to risk your own family but think of the rest of us on the road. Airports are more expensive to rent from but they also have a greater variety of cars and you are more likely to get what you want there. I'd pick up the car and drive to a place nearby for the afternoon and night and start the main trip the next morning. Senlis is not exactly on the way, but it is nearby and it is beautiful:
https://janettravels.wordpress.com/2010/06/12/moyen-age-visit-to-senlis/
Or pick something that is on the way for a rest and overnight.

I strongly advise not to drive the day you arrive.

While I would normally disagree with that as an absolute rule, in this case I think it's good advice. First of all, it sounds like you're unfamiliar with the area. Secondly, it will late in the day by the time you get going. And thirdly, it's a fairly long drive. I think Janet has good advice. Pick up the car at the airport and drive a short distance for an overnight stay. Senlis is a great city, although it is only 20 minutes from CDG.

Arriving at 5 PM, it could easily be 7 or later before you are ready to leave CDG. My advice would be to stay overnight at an airport hotel, get a good night's sleep, picking up the car in the morning, then setting out for Normandy. Driving from the airport to Normandy is relatively easy, but I would advise against it after a long flight. Leaving in the morning would also leave you some time to stop along the way, perhaps Giverny.

There's no direct train service between CDG and Normandy without going through Paris. Like others, I'd advise against driving right after an overnight flight. But landing at 5 pm suggests that you may not be flying overnight from North America, but maybe from elsewhere in Europe, therefore without jet lag?

I'd suggest an airport hotel, or one in nearby Roissy (I don't know Senlis) that first night, then get the car in the morning. I couldn't find a car rental location in Roissy but there may be one, and certainly there are at the airport. It's true, renting at the airport will cost you a little more than renting elsewhere, but likely less than the multiple train fares you'll pay otherwise.

If it's me, I'd train to Normandy and pick up your car the next morning. I stayed in Paris a weekend before taking a train to Rouen and picking up a car there.