It's never Ok to leave your children alone.
I don't think it's helpful to compare these cases at all. The only link is that the McCann case may have inspired the Shannon Matthews hoax.
My take is that the McCanns should never have left their children alone and now they are paying the ultimate price - which in my view is punishment enough (but not much consolation to poor Maddie)
The Matthews crew seem to be a bunch of hopelessly sad and irresponsible individuals who on the face of it seem to be the architects of their own misfortune but in reality are probably the inevitable fall-out of Thatcher's policies in the north of England nearly thirty years on.