It is probably a legal requirement for all children to be wearing PFDs, so the fact that they can't swim isn't a huge issue. The children will have a fair amount of supervision, and the adults should be more than capable of plucking any that fall in the water out with relative ease.
Most modern PFDs are pretty comfortable, and if your children can be trusted to wear one, and not take it off, then I don't really see a problem with them learning to sail before they can swim. Some children won't, for whatever reason, wear a PFD all the time and they should probably learn to swim before learning to sail.
Also, you have to remember in Canada (based on the website url), the water temperatures may be quite cold compared to the air temperatures, and even if a child could swim, the shock of entering the cold water suddenly might make it difficult or impossible for them to do so properly. Wearing a PFD is really the only option—regardless of the child's ability to swim or not.