St. John Chrysostom instructed parents to make the sign of the cross on their children's foreheads as a protection against the devil.
Parents innately worry about their children’s well-being, doing all that they can to protect them from the dangers of this world.
St. John Chrysostom urged parents to extend that desire to their children’s spiritual well-being, protecting them from the very beginning of their life from the snares of the devil.
In particular, Chrysostom explained in a homily on First Corinthians how parents should “seal” their children with the sign of the cross.
[Parents] ought to invest the child with nothing else, save the protection of the Cross … you should inscribe on his forehead the Cross which affords invincible security; do you forego this, and cast yourself into the madness of Satan … from earliest life encompass them with spiritual armor and instruct them to seal the forehead with the hand and before they are able to do this with their own hand, do you imprint upon them the Cross.
The cross is among the most powerful spiritual symbols in Christianity, as it reminds Satan of his ultimate defeat.
Whatever you do, make sure to seal your children with the sign of the cross, starting in infancy and teaching them when they get older how to seal themselves with a powerful piece of spiritual armor.