First of all, as a former teacher, in regard to the chaperoning situation I would say, When you are the chaperone your rule rules. Before going, everyone should be clear about what the rules and expectations are, but "when in Rome" applies here. If you use corporal punishment (which, based on other postings here I doubt, but if you do, make sure other parents know it before they agree to send their child with you). Then, deal with it how you deal with it.
In regard to other situations
My daughter has two older cousins that are, shall we say "undisciplined" so it was especially tricky. The cousins are by-way of an in-law and her parenting style at family functions at least is pretty much "leave well enough alone." She let her two year old slap the newborn all the time when the second one was an infant and said, "Well she is expressing her frustration about the disturbance the baby has made in her life." Obviously, talking to the parent wasn't a real option for me.
This is how I dealt with it
I personally try to model the idea that everyone is different and does things differently. "Well that mommy has a different rule than I do." Period. I might privately discuss it later with my child so she understands what I value that makes the rules for me different (at six, she definitely gets this, at three it was a little tougher, but not as much as one might think). Obviously, with the cousins, I have to be careful to explain without getting critical.
You CAN help your child learn how to care for themselves somewhat
Right off the bat, I taught Alice "I messages" so she could stick up for herself (to some extent). She could YELL LOUDLY, "I don't like that! Stop. I don't want to. . . " This is just a good safety device in general for kids to use with "tricky people" formerly known as strangers that want them to go somewhere without guardian AND with schoolyard bullies because it draws attention. When Alice said something like this, I could intervene with it being incredibly obvious why I was intervening to everyone around.
First, if I hadn't seen what happened, I asked. I let both kids tell me their side of the story. This gave me great information to use with my daughter later if we needed a follow up discussion - either about different choices she could make to help herself out of a situation sooner, or about different choices she could make altogether (like sharing nicely).
If it was bullying or some other kind of behavior like that and a parent wasn't around (or responding) to the negative behavior, I simply repeated my daughter's statements to the child in question if the behavior wasn't already stopping, "Hey, She said she doesn't like that. You need to stop." If I needed to, I removed my daughter from the situation and found something else that was fun for her to do in another space.
Dealing with "Its not Fair"
In terms of the "its not fairs" in life that crop up as a related issue, I simply respond with "fair is a place we go to see farm animals, ride rides and eat cotton candy." What is "fair" is not always what is equal and "fair" doesn't generally exist in life most of the time anyway. I listen to Alice's feelings about something first, empathize and if there is an area where a compromise can be made, I'll make it, but, at different ages I have also offered up different examples of how "fair" doesn't really matter and isn't realistic to attain.
An example I used recently went something like this, "well, Will it be fair for them when they move out of their house and don't know how to do chores to keep it clean and cared for? - Ok, now go pick up your toys. and then come back and you'll need to do one household chore with me. That one was critical of the other, but it is really the truth.
Finally, I'm going to add this, because I originally forgot to address the question of Other kids being a poor example:
My attitude with Alice (and with my former students) was - "The wrong others do doesn't make your wrongs less wrong." Alice knows she expected to stand out in the crowd as the leader who makes the right choice and sets the example for good behavior. I simply won't tolerate less. She also knows that she can come to me
and ask for help, advice, feedback etc. without me being angry with her or judgemental about her not knowing what to do.