DS has never been a thumb sucker but now he is seeing other children sucking their fingers at nursery and asking about it. I personally discourage him from having fingers in his both as I think it's quite disgusting (not judging others who do t for security but that's not why my DS is doing it). So I just say yuck take your fingers out of your mouth. But he's telling other children to stop doing it and telling them it's yucky. One parent was really unimpressed the other day when he went up to a friend who was obviously feeling a bit insecure and had all his fingers in his mouth and DS told him it was a yucky thing to do.
I guess it's a common problem telling your DC not to do something that others seem to be allowed to do.
How do you handle it?