My daughter is still talking about this after four days so I thought I'd pose the question to you ladies.
Four days ago my SIL was over with her two daughters, her youngest and my daughter are both 10 and the best of friends. Well, I reminded my daughter to do her chores, which includes emptying the trash can. We have a dual trashcan. It has to bins and when you put a new bag in it, you poke part of the bag through a hole at the back of the bin then put the lip of the bag around the trash can and pull tight on the part you poked through the hole. Well, my niece wanted to help my daughter so they both took out a bag of trash and were both putting a new bag in. My daughter kept trying to explain how you do it and my niece just ignored her. If you don't put the bag in just this way then it slips down into the bin the first time you put anything in it. So my daughter was really stressing that you needed to do it this particular way. She wasn't being rude but after the fourth time my daughter tried to explain it and my niece just brushed her off and told her it wasn't a big deal my daughter got upset about it.
My SIL stepped in and told my daughter that she should just say thank you when someone is trying to help you and my daughter responded with, "But she's not really helping me if I just have to fix it because she won't do it like I told her." This irritated my SIL who gave me this look of "Are you going to say anything to her?" So I just said to my daughter "Don't worry, honey, I'll fix it later. It's still polite to say thank you." She grudgingly thanked her cousin but for a couple days now she's brought it up that she didn't think she should have thanked her cousin and that if you're going to help someone you should listen to how to do the job and not ignore what you're being told.
I don't think it's a big deal, but as far as etiquette goes, what do you do in that situation? Thank the person or tell them they're doing it wrong and try to help them learn how to do it correctly?