I think looking for a "personal police" is missing the point.
   When I was a child, and "misbehaved" ... [[[ do we even agree children
   do??  If not ... well, happily I'm too old to ever have to live around
   your children ]]] ... when he would scold me, often he would say "I
   don't like being the policeman"
   His point was until I (as a growing child) developed my own sense of
   what is right/wrong, he would happily provide that guidance for me --
   which, as a parent, is his obligation/prerogative/joy.
   As is being done - exploding this Child Supervision thread into the
   perennial "Nanny State" argument does have one merit.  If an individual
   fails to adhere to guidelines of their Society, they are punished.
   That is what Society is for.  The Nanny State advocates would like to
   see members of Society be given greater guidance by the State so as to
   avoid those excursions of acceptability.  The detractors believe
   everyone has the same intuited/divinely inspired ability to adhere to
   Society's laws on their own.  Both miss the others point, as has been
   done in this thread.  That is, all people up to a certain age must have
   guidance and after a certain age; must have the chance not to have that
   guidance.  With an appropriate period of transition.
   Quick question for those claiming "let the child alone, she'll be
   happier on the Internet without muddling parents."  ... do you have
   dependents?  you taught to drive?  by giving them the car keys and then
   going on vacation?  and now they have perfect driving habits?  Which
   they miraculously learned on their own: no doubt YouTubing Mario
   Andretti, and reading that one wikihow page.