You all know adults who undermine your authority when you're out and about with your children. They coddle your kids when they misbehave, and your kids know it and take advantage of the situation to see how much they can get away with, especially when your focus is elsewhere.
I want this to be a relatively generic question if possible, but for one example, my son does this thing where he waits for his little sister to pick a toy, then instantly he wants that toy and none other, even in a room full of toys, and even though it didn't interest him until it interested her. He then starts pestering her about letting him play with it.
He knows we don't tolerate that pestering, and if he came to us we would make him play with something else, so instead he approaches a sympathetic adult, puts on a sad expression, and starts whining about his sister not "sharing." Those adults then treat him like he's the victim, give him lots of attention, and spend a lot of effort finding him a toy he will like better. If we comment on his attitude, they take his side.
First of all, why do adults do this? Second, what are some ideas to either stop it quickly or teach kids not to take advantage of it?
I'm not talking about grandparents or other relatives here. With those people we have had plenty of time to lay out our expectations. I'm talking about people like receptionists, nurses, retail workers, waiters, etc. with whom we don't have time to have a lengthy discussion on parenting.
Consider that our situation might be worse than other parents. My 10 year-old has cerebral palsy, but we still expect her to behave to her potential, which others are not usually aware of. However, it is not in her nature to intentionally take advantage of her situation.
My seven year-old is quite small for his age, and since he is homeschooled, is often out when other kids are in school. I think this leads people to unconsciously set their expectations lower, as if he were preschool age, which he can kind of pass for. He's really the one who milks it for all it's worth, usually at the expense of his sisters.