It would be a given that the students will be wearing PFDs during the lessons so I can see that swimming capability not necessarily be required. Of course a comfort level in the water will be very helpful but your children will not sink like stones since they will be wearing PFDs.
Our son grew up sailing, learning to sail in an opti-style boat starting around age 6 or 7. He has never taken to swimming (but he can stay afloat for a bit). Today at 24 he is an avid racer, already a boat owner and sails recreationally and competitively about 4 days/week on average. He wears his PFD pretty religiously still (a good Mustang inflatable collar) but the lack of true swimming capability has not held him back. If your children are interested, I can only say go for it!!
Ditto the advice on a quality, comfortable PFD. Gone, thankfully, are the days of the Kapok filled mattresses we had to wear as kids!