I was wondering if having someone on your knees is to be understood as having them sitting on your lap. It seems weird to me that Ephraim and Manasseh would sit on Jacob's lap when they were around 17 years old.
I take that they were around this age because 1) they were born before the famine (Gn 41:50), 2) Jacob lived in Egypt seventeen years (Gn 47:28) and 3) Jacob was ill and near his death (Gn 48:1,33) when get to Gn 48:12:
Then Joseph removed them from Israel’s knees and bowed down with his face to the ground.
Maybe it is a custom I'm not familiar with, like the "putting the hand under someone's thigh" thing?
On the other hand, it really seems that they were children because Joseph "removed them" from Israel's knees, like a father carries their children from here to there. So maybe Jacob is not as near death as I supposed after all?
I don't know, both interpretations have their difficulties.