Perhaps one way that we can show mercy is how we deal with a child who comes to us with wrongdoing, as opposed to us finding it out and coming to him. In these cases, discipline has already begun, and I should not discipline as harshly as I would had I discovered the problem.
For example, a child who comes to me and says, “I lied” is disciplined differently that if I discover he has lied, and he tries to cover it up.
See Proverbs 28:13 He that covereth his sins shall not prosper: but whoso confesseth and forsaketh them shall have mercy.