Our son started sailing lessons two years ago when he was 6 years old and I cannot say enough about the experience. Firstly, I think your kids will have (should have) very close supervision and I don't think the instrucutors would let them out unless conditons are ok. I would say our son had weak swimming skills when he started but was certainly comfortable jumping off the dock with his life jacket on. I think any time you can get your kids around water in a supervised environment is excellent as it will build their self confidence (especially with no parents present which sometimes hinders their growth).
In his first year, our son sailed by himself across the lake and back (about 1 mile each way) and this in itself with a tremendous boost to his self confidence.
I suggest you go down and meet the instructors and see what they are like, discuss your concerns and then just go for it!!! You won't regret it.
Post back and let us know how it goes.