Indeed, the UN does not seem to have any specific agreements with Canada, which they have or had with some other countries. Canada is just not a next-door-neighbour to any major UN operation. But the concept is the same, indeed. My guess is perhaps they should look at the situation ad hoc.
We tried asking the Canadian Embassy, will see what they say. But being put on right track would be ideal.
School fees would not be a problem, as my husband's employer pays for them. So a study permit for my five-year old son could make sense, if they don't think it's odd that he decides to study in Canada at that age and I have to be with him... It seems a rather odd way of getting what we need. Otherwise we could even nominally give both kids to my mother for temporary foster care, so they can be there and in school, while I could be there as visitor, but again that's an odd way of doing it.
So, any other thoughts welcome.