http://revach.net/article.php?id=3879

Section: Shabbos   Category: Minhagim
Friday Night Bracha For Your Children

The older brother of the Maharal MiPrague, who was known as on of the great gedolim of his time, writes in his Sefer HaChaim that a father should give a Bracha to his children, both boys and girls every Friday night. One reason is because the Satan who instigates us is not around on Shabbos and therefore the channels of bracha are open and unobstructed. Another reason is because sometimes a parent gets angry at a chid and may curse the child. By giving a bracha first, the subsequent Klala cannot take effect.

Rav Yaakov Emden writes in his Siddur that the bracha should be given while placing both hands on the child's head. This is how the torah says Moshe gave a bracha to Yehoshua and this is how the Kohanim give Am Yisroel a bracha. The Ma'avar Yabok says that the bracha should be given with one hand and the Noheg Katzon Yosef of Minhagei Frakfurt says that the mekor for this is Yaakov who put one hand each on Menashe and Ephraim (Minhag Yisroel Torah).

The Eved HaMelech brings from the Reishis Chochma that the hand has 14 joints (gematria of Yad), three for each finger except the thumb which has two. Including the hand itself it has 15, which is the number of words in the bracha of Yivarechicha. Therefore we place a single hand on the child's head and say that through these 15 joints the 15 words of the bracha of Yivarechicha should apply.

The Minhag Yisroel Torah notes that there is also minhag by some, brought down from the Rav Menachem Mendel of Rimanov, not to give a bracha to the children on Friday night.