I spent 2.5 weeks in Paris last summer with my 7 year old daughter. We spent most of our time wandering around the city, going to local parks, and eating as much bread and pastries as possible!
Her favorites were...
Eiffel Tower. Bought advanced tickets to the top at sunset one evening, and then returned 2 other times for picnic dinners outside. She and my husband kicked balls around with another family and she ran around with kids. It was great!
Louvre. We were not planning to take her but she asked to go see Mona Lisa. We went on an evening it was open late and hit all the biggies plus the Egyptian art. She loved the mummy cats.
Luxembourg Gardens. Playground, carousel, wide open spaces to run, and several ice cream stands.
Museum of Natural History. In the 5th. Very cool museum. Mostly French tourists. Close by is the largest Mosque in Paris, which she also loved.
Doll museum. Cool collection of vintage to modern dolls. It was only okay for me, but she enjoyed it.
Disneyland. We took the train and spent 1 night and 2 days at Disneyland. Besides the ET, this was the highlight of her trip and something we planned just for her.
What she did not enjoy....
Seine boat ride. I thought she would like this, but nope. She bored even though we sat on the top deck. She loved walking along the banks though.
Notre Dame. After a few oohs and ahhs and lighting a candle for Joan of Arc, she was bored. But loved the playground outside and all the pigeons.
My advice is to not schedule too much and just enjoy the city with your grandkids!
One other tip: we had several conversations with our daughter about the heightened security in Paris (we were there a few days after the attacks in Nice) but it was still a bit of surprise for her to see police officers armed with machine guns everywhere. Just wandering down the streets on patrol. After a few days she was used to it, and by the end of the trip was saying "merci" to the officers.