It is an old custom for the parents to bless their children on Shabbat night after Shabbat prayers. (Mentioned in siddurim from the early Acharonim.) The blessing for the sons begins “May God make you like Efrayim and like Menashe”; this is a fulfillment of Yaakov’s blessing to them in our parsha: “And he blessed them on that day, saying, [the people of] Israel will bless by you, saying, ‘May God make you like Efrayim and Menashe'” (B’reshit 48:20).
The Sefat Emet points out that this is a very cryptic blessing. Seemingly Efrayim and Menashe are not the most blessed among the Jewish people, or even among the tribes. On the contrary their blessing is itself a secondary one, for the primary blessing Yaakov granted them was that they should be equal in status to the other tribes: “Efrayim and Menashe will be to me like Reuven and Shimon” (B’reshit 48:5). Seemingly it would be a much more elevated b’rakha to tell the sons, “May God make you like Reuven and Shimon”, who are the models for Efrayim and Menashe!
The Sefat Emet then explains that this is precisely the value of the blessing: “For since they were the generation after the tribes and even so they rose to be among them. And this is because of the blessing, to be joined to the root [of existence] even though they were on a lower level.
And in this way blessing can be drawn down to each member of Israel from the holy Patriarchs, even to a thousand generations, for the root is from them.”
If it were not for the blessing of Yaakov to Yosef’s sons, we would not be able to bless our sons that they should be like Reuven and Shimon. The level of blessing of the original sons of Israel is quite beyond us. It is precisely because Yaakov, last of the Patriarchs, blessed the sons of Yosef who were not at this level that they should be like Reuven and Shimon that we in turn are able to bless our own offspring that they should ascend to the level where they will share in the blessing of the Avot and the original tribes.
In this way the Sefat Emet answers another conundrum. As Yaakov places his hands on the two grandsons and begins his blessing, the Torah tells us: “And he blessed Yosef, saying…” (B’reshit 48:15). Yet the blessing is not to Yosef, but rather to his sons! The Sefat Emet explains, “Since his sons ascended to be like the tribes, automatically he ascended even more”.
The highest level of blessing is the ability to transmit blessing beyond ourselves to the next generation. (Sefat Emet Vayechi 5632)