Personally, I think Quebec city is a much better place to visit, though Montreal is lovely. As others have said, have lower expectations when traveling with kids and you will be OK. Visit parks every day, find fun eating places and daily ice cream. Plan one touristy thing every day rather than packing it in. Travel on interesting modes of transportation, like buses and trains and verniculars.
We did that exact itinerary when the kids were little, so I would be happy to share some ideas.
I’m sure it is overwhelming as a single parent, but it will be a great memory for your family.
"Friendship is born at that moment when one person says to another, "What? You, too? I thought I was the only one." C.S. Lewis