I think the summer/winter difference is a combination of how Dave explained it, and what Cat Herder said:
Originally Posted by Cat Herder:
My personal opinion is that families have many more scheduled activities to juggle in warmer months than in colder ones. Sports, end of school activities/testing pressures, camps, recitals, etc.
Add to that the commercial focus on family time and photo op holiday gatherings keep the parents' attention more on the kids during the colder months.
People do forget their kids in the winter too, but probably not quite as often, and there would be more time to be discovered before it's too late.