I would drop off in Paris because having a car allows you the freedom to see more in a very short period of time. You will get a lot done, giving you greater value for your money spent on the trip. Just my $.02
Some will steer you away from dropping off in Paris. Driving IN the main center of Paris is gutsy. We did it, both during night and day times and didn't suffer a dent or a ding - even during rush-hour. But then we are from Miami and here they drive insanely albeit nothing like Paris.
The drop off isn't exactly in the center of Paris and it's at the airport. We washed the car right behind the drop-off point, dropped off the car, did the paper work and were shuttled to the terminal by TT employees for free in less the 45 minutes.
We really enjoyed seeing three cities (Reims/Epernay, Strasbourg, Meinz) in the course of a long day, I remember. However, your child will need to stretch those legs so perhaps I'm not one to listen too. I know that having the freedom to stop and entertain a young one is easier to do with a car than waiting on trains and in them!
Best of luck and have a great time. BTW, we dropped off the car at TT in Paris and it took 6 days to get from that drop off in Paris to Bremerhaven. It was in Bremerhaven for 9 days and then it was loaded onto the ship. One more day passed and then it sailed. We now are awaiting our re-delivery at the PCD but the ship is supposed to dock on the 24th of September, this month. So, expect it to take two weeks to get from the Paris drop-off to on a ship that is sailing, if you drop off in Paris.