We went at Easter - our dc are 11, 9, & 6.
If you can book a time slot to go up the Eiffel Tower, do. Horrendous queues. We climbed the stairs to the second floor, 6yo managed fine.
Louvre massively crowded so we went to the Mona Lisa, the Venus de milo, the Egyptian sphinx, the French Crown Jewels and the cafe
. Told the kids what we were going to see and then went hunting for it.
Boat trip on Seine was great. They quite liked the metro too ...
Notre dame - we ended up not going inside; I think you have to get there at the crack of dawn to avoid the miles of queues.
We got the funicular railway up to the sacre coeur at Montmartre - no queues there so we went in (are you sending a theme?) and then there is a road train that takes you round Montmartre - because by then we'd had enough of walking.
We went up the arc de triomphe too, that was cool.
And we went to lots and lots of bakers and cake shops and cafés. Yum.
The paris museums pass is worth looking into - you get to queue-jump.