Objection 1. It would seem that penance cannot be in the innocent. For penance consists in bewailing one's evil deeds: whereas the innocent have done no evil. Therefore penance cannot be in them.
Objection 2. Further, the very name of penance [poenitentia] implies punishment [poena]. But the innocent do not deserve punishment. Therefore penance is not in them.
Objection 3. Further, penance coincides with vindictive justice. But if all were innocent, there would be no room for vindictive justice. Therefore there would be no penance, so that there is none in the innocent.
On the contrary, All the virtues are infused together. But penance is a virtue. Since, therefore, other virtues are infused into the innocent at Baptism, penance is infused with them.
Further, a man is said to be curable though he has never been sick in body: therefore in like manner, one who has never been sick spiritually. Now even as there can be no actual cure from the wound of sin without an act of penance, so is there no possibility of cure without the habit of penance. Therefore one who has never had the disease of sin, has the habit of penance.
I answer that, Habit comes between power and act: and since the removal of what precedes entails the removal of what follows, but not conversely, the removal of the habit ensues from the removal of the power to act, but not from the removal of the act. And because removal of the matter entails the removal of the act, since there can be no act without the matter into which it passes, hence the habit of a virtue is possible in one for whom the matter is not available, for the reason that it can be available, so that the habit can proceed to its act--thus a poor man can have the habit of magnificence, but not the act, because he is not possessed of great wealth which is the matter of magnificence, but he can be possessed thereof.
Reply to Objection 1. Although the innocent have committed no sin, nevertheless they can, so that they are competent to have the habit of penance. Yet this habit can never proceed to its act, except perhaps with regard to their venial sins, because mortal sins destroy the habit. Nevertheless it is not without its purpose, because it is a perfection of the natural power.
Reply to Objection 2. Although they deserve no punishment actually, yet it is possible for something to be in them for which they would deserve to be punished.
Reply to Objection 3. So long as the power to sin remains, there would be room for vindictive justice as to the habit, though not as to the act, if there were no actual sins.
Source: Summa Theologica (New Advent)