Blessing of the Children V: Yosef Ometz
It appears to me that the idea behind the blessing of a Tzaddik is to draw the blessing from heaven to the one who is being blessed by opening the recipient’s pipes to heaven, his soul and his ability to receive from God.
This is why we bless our children immediately after returning home from prayer. At that point, we too, are connected to receive Divine Abundance and are better equipped to help our children receive the same Divine Influence.
Although I do not know anything about hidden matters, I suspect that it is better to bless with all ten fingers placed on your child’s head to hint to the Sefirot, each a separate pipe of Divine Influence. The parent should keep this in mind when placing his hands on the child.
Both hands also indicate that the parent wants to give all he has to his child. (Yosef Ometz of Rav Yuzpah, the student of the Terumat HaDeshen, #70)