Truthful Children

In the film Cosby mocks Childless adults for saying children are truthful.

I don't think they mean kids don't lie. I think they mean that sometimes kids don't know what's wrong to say out loud.

For instance, in the Cosby Show series finale, the family is going to a College Graduation and Cosby's character reviews with his grandchildren what they will not do in public (like pointing out a man's ear hair)

That's what childless adults mean by truthful children. They point out a man has ear hair while more refined adults would leave that a secret