Funny thing is that some parents want you to step in. My first time working with younger kids I was a Cub Master of a new cub scout troop. Parents at this time needed to attend the meetings, I felt very uncomfortable enforcing good behavior on other parents children, espesially when they where in the room. On one outing I had a new parent (He was also a Middle School Principal. He had no problem stepping in, and the parents where thankful. Big shock to me, I now am more assertive, with rarely any issues. However I haven't tried it with strangers.
