Based on what you said, I’d actually suggest NOT staying in the Old Port area. It’s easily accessible via metro and can be very crowded, with some only so so restaurants. Instead, stay near La Fontaine Park or Le Plateau Mont Royale. They remind me a lot of some NY neighborhoods with old rowhomes and lots of gorgeous green space, cute shops, tons of great bakeries and restaurants, and great walkability. My kids loved going to the local bakery and playing at the park with some local kids, who practiced their English with us.

Must sees with kids are the Science Museum, the Biodome (not the Biosphere— don’t get them confused!), and Mont Royale. Depends on how much time you have; could also do a tour of the St Lawrence or go out to Mont Tremblant.