I think there's more to this story. Ages 3 and 5 are not too young to go out for a day sail in decent conditions, if the skipper is reasonably competent. Though they didn't comment on whether they were wearing PFDs, but is seems likely they were not, since the water temperature at the surface was 72 degrees. I noticed that on July 13, the boat had a mast up. But, the recovered boat did not even seem to have one present. Though the boat was listing in the water, it was still afloat and there is plenty to hold onto.
It seems unlikely that the boat simply capsized, resulting in all three drowning unless this played out in the most tragic way, with the father drowning trying to find his children.
We know that they beached on July 13 (from the photo). So, if I had to speculate, when they left that beach, the father simply forgot to put the keel back down.