Remember that we do not measure the age of a soul like we measure the age of human beings.
St. Paul, St. Catherine of Siena, and others have taught us that the soul is in a persistent state of dialogue with God. Cooperation with grace can make this dialogue, this embrace of the divine life, more fruitful.
It sounds sad to say “a child can go to hell”. But the fact is: a soul that dies unrepentant of mortal sin has chosen to reject God. It is their choice, not God’s. God does not “send souls to hell”. Souls choose. That is why life is a process of “conversion” (turning towards God, choosing God).
That is why the Lord Jesus Christ, who was like us in everything except sin, left us the Sacrament of Confession. When we commit mortal sin, we go as soon as possible to confession. There, Christ dispenses the forgiveness He earned for us by dying on the Cross. It is a good habit to go to confession every two weeks or once a month, and to do a good examination of conscience once or twice a week.
Some spiritual writes recommend that the more advanced in the devout life do it daily and confess weekly, but that is not for children. However, every seed must be planted and watered. If we want our children to grow in true devotion, we must begin by helping them with the basic. A good habit, just like a bad vice, is hard to eradicate.
So yes, it could happen. But keep in mind that committing a mortal sin is not
that easy, and that we have the undeserved blessing of the Sacrament of Confession.