All matters of impurity are mortal sins. You should not think that just because your child is young there is not mortal danger to these sins.
You must pray and use strong discipline, physical as necessary. You must pray and make use of every grace.
‘Let us not doubt that baptized babies who die in their infant years will enter into the heavenly Kingdom. We should not, however, believe that all those infants who have begun to speak will enter the Kingdom of Heaven. For the entrance to the Kingdom of Heaven will be closed to many babies because of their parents’ bad rearing. In this city, there lives a certain man who is known to all; three years ago, this man had a son who, if I recall, would then have been about five years old, for whom he had such human love that he did not even try to discipline him.
For this reason, the boy, when someone prevented him from getting his way, used to blaspheme the magnificence of God—and let me emphasize that this is something dangerous.
When, three years ago, a deadly plague fell upon the region where he lived, this young boy succumbed to it and was near death. As eyewitnesses recounted, while the father took the child into his arms, the boy himself saw evil spirits coming for him. The boy began to tremble, to blink his eyes in fear, and to cry out in despair to his father: “Father, save me, protect me.” Simultaneously, as he cried, he turned his face towards his father’s chest, as though wanting to be hidden.
When the father saw his son trembling, in agony he asked him what he had seen. The son answered: “Black creatures came to me and wanted to take me away with them.” No sooner had he finished this phrase, than he immediately blasphemed the name of the Divine Magnificence and, with this blasphemy, expired.
Thus, God, the All-Powerful, in order to show by what sin the boy was given over to these evil servants, allowed him to die with this sin which his father, while the boy was alive, did nothing to prevent. And this boy whom God allowed, by His mercy, to live as a blasphemer, by His righteous judgment was also permitted to blaspheme at his death, so that his careless father might know well his sin. For this father, being indifferent to the soul of his young son, reared for the Gehenna of fire not an insignificant sinner, but a great sinner.’
Pope St. Gregory the Great