Charles Ellicott Commentary Genesis 50:23

"And Joseph saw Ephraim`s children of the third generation: the children also of Machir the son of Manasseh were born upon Joseph`s knees." — Genesis 50:23 (ASV)

The third generation. — These would be Joseph’s great-grandchildren. Thus Eran, son of Shuthelah, son of Ephraim, was to be born in Joseph’s lifetime (Numbers 26:35–36).

Were brought up ... — In Hebrew, were born upon Joseph’s knees, that is, were adopted by him. (See the note on Genesis 30:3.) They would not form tribes, as this prerogative was reserved for the sons of Jacob (Genesis 48:5), but they would count as Joseph’s sons (Genesis 48:6), and form “families.”