Some ideas of things to do with children while in Montreal:

• La Ronde (open on May 31, June 1 & 2) 
• The Biodome, 
• Insectarium
• Botanical Gardens
• Planetarium
• Visit the Old Town
• Redpath Museum (closed on Saturdays)
• Go for a nice walk at Mount Royal Park
• Ride the métro (subway) and discover the different stations
• Other ideas: http://www.where.ca/blog/kid-stuff/50-things-to-do-with-kids-in-montreal/