Genesis 50:23 - Matthew Poole's English Annotations on the Holy Bible

Of the third generation, reckoning from and after Ephraim, i.e., Ephraim's grandchildren's children. So early did Ephraim's privilege above Manasseh appear, and Jacob's blessing \Genesis 48:19\ take place. The children of Machir, Heb. sons. For though he had but one son, viz. Gilead, by his first wife, yet he married a second wife, and by her had two other sons, 1 Chronicles 7:16, which Joseph lived long enough to see. Or under the name of children his grandchildren also might be comprehended. So there is no need of that enallage of sons for one son which we meet with in other places. Were brought up upon Joseph's knees; laid upon Joseph's lap or knees, where parents use ofttimes to take up and repose their infants, to express their love to them, and delight in them. And some observe, that it was an ancient custom in divers nations, that the infant, as soon as it was born, was laid upon the grandfather's knees. So it is an ellipsis, whereby one word is put for two, or under one verb. See more of this phrase on Genesis 30:3, Genesis 48:12.