if you are coming later in month this old style market is of interest to kids as have different activities, some food.
You might get some picnic food for lunch or some croissants, buns for breakfast and enjoy the old montreal scene, bikes, quadricyles, pedalo boats, taxi boat that crosses river and back or even more fun amphibus, caleches. There is also muvbox near the locks and silos.
The small china town is an interesting place for stroll on Sundays and lots of places there for shared meals
The sandwiches at Santropol, a smoked meat tour going from Schwartz's to the Main to Lester's. Amelio's pizzas, the Binerie Mont-Royal, La Flammee for the flammekeuche, the Noodle Factory, Qing Hua for dumplings and Wilensky's Light Lunch are all extremely kid friendly, not expensive and very delicious.