Interesting, intimate museum.
The Redpath Museum is a small three story museum on the McGill University campus. The buildong itself is safe, sturdy, and... Dated isn't the right word, but in need of some updating to help woth flow. It has an eclectic mix of geologic, biological, and human history/science. The museum itself is free, but a donation is nice. 20$ CAD for a family. The signage is minimum but sufficient. You can expect to spend about an hour here.
I have a 10 and 7 year old and they both enjoyed the museum. It is a convenient stop before or after hiking/visiting Mount Royal. There are also plenty of places to eat lunch, dinner or snack around the campus.