I've never had any children but of course I have a right to comment on how children are brought up. We all have to live with those children in society and then with the adults who are a product of their upbringing. It's those children and adults that make the society we live in so of course we can comment on what and who is being produced by their parents.
If we all have the right to comment on how people bring up their pet dogs we can comment on their children!
The posts about children sneakily stealing sweets sound strange to me as my daughter is exactly the opposite bless. She has one of those dairy milk money boxes in her room and a bubble gum machine and could quite easily stuff her face with them but she doesnt. She comes downstairs and asks me if she can have one! If I say no then she wont get one.
You could argue - if you were bothered enough - that it's unreasonable of your friends to discuss child-rearing issues in your company, since you don't have any - but yes I know what you mean, I have in the past been told that I don't know what I'm talking about, since we don't have children. It's not entirely relevant, as others have said we've still had a great deal of experience with other people's, friends and family, and you can pick up a lot by osmosis!
Of course you have a right to an opinion, but you're probably risking life and limb if you dare express it without being asked. Even if a parent has the most objectionable and badly behaved child imaginable, they will still think their infant is the perfect little darling, so best to keep silent unless they specifically ask for advice.