Tons of stuff!! Here's a list of stuff to do that a kid might like:
'Touristy' 1. The Eiffel Tower - Always a fun place to go. The top tower is well protected ie: you won't need to worry about your 6 year old falling off by climbing between the rails. At 600 feet + tall, it offers a great view of Paris, along with telescopes to see various places close up. Also, The Museum of Man is very close - just a short walk from the Eiffel Tower.
2. Le Champs-Elysee / Arc de Triomphe - The road between the Eiffel Tower and the Arc de Triomphe is named the 'Champs-Elysee'. Although a big-time tourist trap, there are many fun shops and restaurants meant to lure you in.
3. Le Louvre - Among the largest museums in the world. Hundreds of years ago, the Kings of France lived in this building. It's hallways are so wide and tall that various Kings of France hunted fox through it's hallways in inclimate weather. Although the grandkids might get bored, this is a must-see for adults. You may need to make several trips to see everything (Statue of David, Mona Lisa, etc).
4. Le Cathedrale de Notre Dame - A beautiful cathedral in the middle of Paris. If the grandkids are patient enough to watch the Hunchback of Notre Dame, they'll appreciate seeing the 'real thing' in person.
5. Boat rides down the Seine - The 'Seine' is the river that flows right through Paris. Many of the sights listed above can be seen by riding a boat down the Seine.
6. Basilique du Sacre Coeur - Located on the highest hill in Paris (Montmartre), in the 18eme Arrondisement, this beautiful white Roman Catholic structure was dedicated to the Sacred Heart of Jesus (Sacre Coeur = French for 'Sacred Heart'). Other than the structure itself, many local artists hang out in the streets around the structure - their paintings on display.
7. Disneyland Paris - Never been there, but the kids might like it.
'Not as Touristy, but still really cool'
8. L'Opera - The Paris Opera house is truly magnificent.
9. Versailles - Although you'll need to take the Subway, the jardins de Versailles (Gardens of Versailles) are legendary! This castle is where King Louis XIV and Marie Antoinette resided. As you probably know, King Louis XIV took Marie Antoinette to wife while she was still a Teenager. As a result, Marie Antoinette had a 'Playhouse' built on the site - the exterior made of pink marble.
10. Chateau Vaux le Vicompte - The last privately owned castle in France is a few miles from Paris. It not only has beautiful gardens, but also provides tours.
11. Chateau Fontainbleau - The residences of Kings of France from Louis VII through Napoleon Bonaparte. This is a little further away from Paris proper (abt 30 miles), but is still pretty cool.
'Other stuff to do' 12. Street Vendors - Take a chance and order Crepes through one of the street vendors. They're really delicious! If you can find one that sells Chestnuts, give them a try. I still think of how delicious those salted, buttery chestnuts were.
13. Boulangerie - If you like sweet stuff, you should not miss the chance to buy stuff at a Boulangerie. From Eclairs to Religieuses, you'll find them to be delicious.
14. Cheese - Don't be afraid to try the local cheese. Every area has different varieties they specilize in producing. When you're at a Boulangerie, pick up a 'Pain' (pronounced 'pan') or a 'Baguette' for your cheese. If you want to start out simple, give Camembert President a chance. If you're a lot more adventurous, you can try Meunster - truly delicious, if you can get passed the smell.
'Things to avoid' 15. The Moulin Rouge - Although you and your wife might like the Moulin Rouge, it's in a 'Red-Light' district that is not very child-friendly. You might have some of those same problems at Saint-Denis.
One Tip - The residents of Paris are actually quite friendly. If you try and engage them in French - even if it's broken French - most will happily return the conversation in English. However, if you don't try speaking French, many French consider that an insult and will try and ignore you.