once:

Does summer vacation really need to be twelve weeks long for kids to go to a one-week summer camp? Why can't they have four of those weeks in the winter?

I would say that summer does need to be long for camp - because most camps are much longer than that, and many kids go to long camps, or multiple short ones. Not to mention that for older kids who are interning or doing community service, the way to get the most out of those experiences is to have a significant length of time to immerse oneself in the work environment.

I don't see any reason why kids shouldn't have a longer vaction in the winter - in fact, many colleges take advantage of extended winter breaks. I just graduated from undergrad and my college required us to complete three winter term projects to graduate. These projects can entail almost anything, from classes to internships to personally designed projects on practically anything. As a result of my winter terms, I got to work at an environmental non-profit, do ecology research in Panama and work for Habitat for Humanity.