Trying to look up train tickets online, it appears we need to take 2 different train lines and stations to get there. Has anyone else done this trip from the airport?
I wouldn't consider it "easy," only because there is no train that leaves CDG and goes directly to Rouen. Most trains to Rouen leave from Gare Saint-Lazare. If it were me (and I have done this many times), I would take the Roissybus from just outside of arrivals at CDG to Garnier Opera (13€ per person). From there it is a 7-minute walk to Gare Saint-Lazare. Alternatively, or if you are more than three people or have a lot of luggage, you could take a fixed-rate taxi from the official taxi stand (ONLY) from CDG to Gare Saint-Lazare (56€ for up to four people). As frequently mentioned, do not accept offers for a taxi within the airport. Only go out the door (follow the footsteps in the halls) to the official taxi stand—the line for which moves quickly.
In June, we took a Flixbus from Rouen to CDG. I think it was about a 2 1/2 hour trip. Very comfortable and easy trip. They have a bus from CDG to Rouen. Flixbus.com