"It is a matter of great importance what intention a man has in showing leniency. Just as it is sometimes a mercy to punish, so it may be cruelty to pardon." (Augustine, Letter 153, section 17, [p. 126 of From Irenaeus to Grotius: A Sourcebook in Christian Political Thought, edited by O'Donovan and O'Donovan])Chew on that one for a while.
Parents sometimes operate as if they ought to spare their children from any and all hurt -- including punishment. We pick up their toys, do the chores they've left undone, and never get around to giving them the punishments we threaten, all because we don't want them to become discouraged or (worse yet!) to dislike us. Eventually we wonder why we can't get them to do anything at all.
The truth is, our kids need to experience real life if they are going to become well-adjusted adults. In real life, people don't clean up your messes. In real life, people don't do your chores. In real life, painful consequences follow bad decisions. If we spare them all this when they are young, they'll spend the rest of their lives thinking that they've been dealt an unfair hand. They'll continue to act like children long into their adult years, thinking that the world owes them something. We see it all the time, and it's not pretty, is it?
Knowing when and how to show mercy is one of the mysteries of parenting. But Augustine is right: mercy and pardon are not the same thing. As the author of Hebrews reminds us,
"Now, discipline always seems painful rather than pleasant at the time, but later it yields the peaceful fruit of righteousness to those who have been trained by it." (Hebrews 12:11 NRSV)