I'm afraid you're out of luck there - public schools in Montreal will tend to teach in French. While there are some public English language schools, these are reserved for children whose parents are Canadian citizens (as you're moving to Canada, I assume you're not a citizen and will be there on some kind of visa).
Some young children may be able to cope with being taught in another language but it's very difficult to say what the upper limit for this is. Some kids will pick it up faster than others, and some may really struggle, especially if they've never spoken the language at all. My advice would be to start working on their French as soon as possible so that they can be somewhat prepared for the change. Another option is that, even if you'll only be there for a short time, it still may be less traumatic to put them in an international school. Expat families are often quite mobile in general, so I would think that some international schools might be willing to accommodate you in this regard, especially if you get in touch with them in advance.
For more info, you can have a look at our page on Education and Schools in Montreal. At the bottom of that page there is also a link to a list of international schools in Montreal.
Best of luck.