Trevor, I, too, liked the "S.E.P. Device" in the Douglous Adams novels. (Things become invisible when they're Somebody Else's Problem.)
But really-- the jealous protection of our children is everyone's problem. If there is a breakdown in our society that we can all agree on, I'll wager that one of them is our collective reluctance to take responsibility for the wellfare of our children. While the parent should have an eye on their child, their carelessness doesn't merit the LOSS of their child. You see a threat, it's your duty as an adult, moreover, as a MAN, to check it out. That's what makes us the grown-ups, now.
And sometimes, it's a big ole pain.