My son wears one of the orange Cal June/Jim Buoy harnesses with the crotch strap. I agree with T37Chef, you should be concerned about your child developing heat stroke. Small children are more susceptible to heat-related illness, and you need to be careful in hot weather. We always watch him very carefully on board.
Our son wears a PFD with crotch strap in cool weather and in all weather on the dock and when he boards and leaves the boat. In the heat of summer, it is harness only once he gets on the boat. In addition to our concerns about his safety and healthy, we also want him to enjoy sailing and feel comfortable on the boat.
Here he is in both, as we motor out of Urbanna Creek: