Depending how old your children are, Le jardin d’acclimatation is lots of fun – park, zoo, train, small waterpark, playgrounds with somewhat different stuff than the usual over here, some kiddy rides, etc. My son is two, and had a great time. In some parts of the park, peacocks are strutting around uncaged showing off. On the other hand, if your kids are older, they might find it small potatoes.
I’m assuming you’ve been to the Eiffel Tower before, but if you want to get up on a building, go to the Tour Montparnasse. It’s excessively ugly, but the view from the rooftop is spectacular. They’ve recently renovated the rooftop, so you can even sprawl out and have a picnic lunch if it’s a dry sunny day. There’s a glass wall around the entire thing that allows great views in every direction. Less of a wait to go up than the ET, and the best view you’ll ever have of the ET itself.
The Catacombs are indeed very interesting, but I guess again it depends how old your kids are. For some people, it’s a tough climb down into and up out of (though maybe that’s changed – I was last there in 2002).
For my money, Le parc des Buttes de Chaumont (in the 19th) is the most beautiful, interesting park in Paris. And it’s not too far from La Villette, which has a huge sort of interactive science/technologie installation for kids (I assume your kids speak French if they’re going to summer camp in France). You could certainly make a (long) day out of these two places. Les Buttes de Chaumont on a sunny day is heaven to me. Bring wine and cheese and baguette.
I haven’t been in a heck of long time (at least ten years), but the massive flea market just north of Paris at St. Ouen was definitely interesting back when I used to go. Far and away different from what I was used to in NA. And not far north of Montmartre if you want to make a day of the two.
Organ concerts at La Madeleine were often quite thrilling.
I’m not sure if it’s still there, but this summer, they had a fifty metre pool in a boat moored in the Seine just outside the BNF. It looked inviting; I wish we’d gone for a swim in it.