Adam Clarke on Genesis 30:3

She shall bear upon my knees - The handmaid was the sole property of the mistress, as has already been remarked in the case of Hagar; and therefore not only all her labor, but even the children borne by her, were the property of the mistress. These female slaves, therefore, bore children vicariously for their mistresses; and this appears to be the import of the term, she shall bear upon my knees.

That I may also have children by her - ואבנה ממנה veibbaneh mimmennah, and I shall be built up by her. Hence בן ben, a son or child, from בנה banah, to build; because, as a house is formed of the stones, etc., that enter into its composition, so is a family by children.