I think people talking about divorces and child abduction are a bit premature, there are many things to consider before you could remotely get to that stage.
While you are asking about schools, I think the first thing for you to consider is whether you will be able to work in Canada (in particular Montreal), in a line of work you want to undertake? And if so, would your legal ability to work be tied to your husband's job?
Allied to that, should it not work out, for one reason or another (ie commercial firms are always changing and merging, and this disrupts employment) would you be able to stay in Canada and both of you find suitable alternative employment?
Realistically, as others have pointed out, starting out with a plan for two years almost inevitably turns into a longer period, or perhaps a relocation elsewhere.
For your schooling plans, I can't help with recommendations for schools in Canada, but it appears that you have relatively good funding, so overall your children should be able to find a school that suits - and living abroad is a good experience for them (particularly if they also learn a foreign language). So, from that perspactive I wouldn't worry too much about whether you are disrupting their education. The disruptive thing would be moving to Canada, and then moving back relatively quickly.
Just try to avoid moving when coming up to important transition stages for your children, ie choices for GCSEs/A-levels/University. Best to move at the point those become effective (ie finish year 11 in Canada, and move to sixth form in UK), or a couple of years prior (difficult with two children).
However, I think a real conversation about this with your husband is essential. He needs to understand what your concerns are, and how you both address them. Possibly start it off as a project for yourself, write down what you think are the items that need to be covered, and research the ways to address those. Then have the conversation. It would be fairly difficult for him to ignore issues when you've gone out and done the research to back up your concerns.
At the end of the day, do not get too stressed about it right now, it is still just a possibility. It may be that he isn't offered the job, and all the angst associated with this will dissipate. But don't be complacent, work out your answers to the questions posed, before they are posed.
You've started that already, by researching schools, just carry that on. Bon chance!