Our daughter began sailing with us when she was 6 weeks old in a car-seat lashed down in the cockpit. At the point at which she was able to walk, we put her in a lifevest with a harness that allowed her to reach, but not exceed the edges of the combings in the cockpit. Since she started out with this arrangement from the get go, she never had a problem with it, just as putting on a seat-belt in the car became second nature. Today, she and my wife both wear inflatable PFD's aboard ship as they manage the yacht, and harnesses whenever we're off-shore, (as do I) without giving the matter a second thought.
Our first rule aboard the boat has always been "Safety First". Our second rule is "Always hold on to the boat". Our third rule is "Never argue with Daddy". S'worked for 20+ years for me--it'll work for you. (Rule 1 and 2 anyway.)
That's just how we've done it with all of ours.