How Much Should We Butt In

The other day my colleague and I were talking about things relating to parenting and raising a child.  And the question came up.  How much should we interfer in another parent’s sphere of influence when we see that parent’s child behaving inappropriately such as slapping their brother/sister or Heaven forbid, slapping their mother.  Do we stand back and whisper among ourselves about how incompetent that parent is.  Do we stand aside and say with some conviction that “If that were ever my child I would….”.  Would we go up to the child and tell the child “That’s wrong.. don’t hit your brother, mother etc”.  Or would we pretend we did not see anything