There is a tendency with some people to grin when they talk about sins they have committed. With some it seems to be out of embarrassment, with others because they somehow think that what they did was cute – especially the sins they committed as children.
Just remember that sin is sin in God’s eyes – disobedience to Him – and all just as bad. Try to communicate to the children that no sin is cute and should be avoided. Teach them not only with your words, but also with your expression. Don’t laugh at what is offensive to God. It will teach the children that some sins are no big deal.