One day when I was up in Montmartre, I stopped at Le Bal Café, 6 Impasse de la Défense, 75108 for lunch. The cafe was started by two former employees of the famous Rose Bakery and the menu has an Anglo-French flair. Although I have heard brunch can get quite busy, I did not have a problem getting a table for lunch. After eating I wandered next door to the Le Bal exhibition space and perused their small bookstore and the exhibit. Check out their website for information on their upcoming shows.
As you can see from the photos, the cafe is a particularly nice spot for lunch or brunch when the weather is pleasant. And, just across the way from the cafe is the Square des Deux Nethes. The park has a sweet community garden, two play structures and even some patches of grass for relaxing. Depending on your comfort level, you might let the kids play in the park while enjoying a coffee or a glass of wine at the cafe.