How did that go? I've done it in a few other places and it worked but those cities are big. Rome strikes me as the better idea, but maybe I'm missing something. I just don't think it would be wise to stand in a long line with a 6 year old at the Eiffel Tower. It's been a long time since I've been there
Well, you don't _have to_ go on the Eifel Tower - if you want a bird's eye view of Paris, Arc de Triomphe and Montmartre are also options.
An evening picnic on Champs de Mars can also be pretty nice - they light up the tower at night. Also, Versailles is great for a day trip with kids.
I feel like there is enough to do in Paris with a six year old, and that's just conjecture. Also, I've never been to Rome, so no idea about how they compare.