This certainly is an interesting edge case, but it's not new one. Established law in the US has been that courts can compel production of the key to a safe but not divulging a combination; this is just a logical extension to new sorts of "safes".
Politicians with stupid policies get elected in large part because the media love to cover idiocy because it gets lots of attention. This causes the mere-exposure effect to kick in, where someone gets voted for merely because his name is familiar (which explains most of the incumbent's advantage).
Mike, for the detailed analysis you do in these cases, you tend to trip into the practice of throwing the word "right" around inaccurately. It's important to hold the government (and cloud-yellers) to what the law actually says, and the Constitution does not claim that the government has any rights whatsoever; rights are held by the people, which delegate specific (ha!) powers to the government.
While I'm usually in agreement with both Mike's analyses and critical of most think-of-the-children measures, I fail to see how it's outrageous for parents or guardians to have the authority to access their minor children's online accounts. They have legal responsibility for their children, and they already have analogous authority to read their diaries, listen in on their phone calls, and monitor their computer usage from the client end. In the cases of actual mistreatment, as noted in the article, children can be legally emancipated, and parental rights are terminated.
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