It has been my experience as a parent that a swimming test is required before sailing lessons begin. I have two children sailing Optimist boats and every year they must swim 50 ft off the dock, retrieve their lifejacket, put it on and swim back. It is state law here in NY that all children under the age of 14 wear a PFD when on a boat. Consider swimming lessons at a local Boys & Girls Club or YMCA this summer.
Just an FYI...The club where my children sail, taught them to "turtle" or capsize their boats. They learn how to get out from under the Opti and to turn it back over and bail. It is an important lesson, because if a wind blows them over it decreases the fear and panic.
Good luck with your decision.