I was recently a child in the uk only a few years ago (I'm in my early 20s) and got into trouble for having a knife while playing out in the woods. I was sent home with the knife confiscated, and it was later returned to me.

Of course, its entirely possible in other circumstances for it to be less/more acceptable. While at scouts we could of course carry knives without worry, and even met police officers who came to talk to us who had no issue with us carrying it while explicitly using them for scouting.