It's not uncommon to have a child come to me to confess they've broken a rule or offended a sibling. When one of you demonstrates the humility and character to do so, I don't want to punish you at all! Unfortunately, if I choose to not mete out any punishment, the confession becomes a "get out of jail free" card. So how do I grace-fully handle a confession? I ask a simple question:
What do you think your punishment should be for this offense?
The child will almost always answer that question with a harsh penalty -- far greater than I would enact even if I had discovered the sin without the confession. This allows me to mete out an appropriate punishment but still show that child grace.
On the rare occasion that a child is "confessing" to avoid getting in more trouble when their offense is inevitably revealed, they'll suggest a powderpuff punishment, and I can do some additional heart work.
Effective discipleship is sometimes tricky, but God always gives wisdom for the task.