Or HaChaim on Genesis 47:24/אור החיים על בראשית מ״ז:כ״ד

1 חמישית לפרעה, one fifth for Pharaoh, etc. Joseph was astute enough to say that Pharaoh would collect the first 20% of any seed. Even if, as a result, the remaining 80% would be only sufficient for the farmer to plant and to break even, Pharaoh would not agree to collect less than his 20%. On the other hand, the farmers would be at liberty to collect the overage whether there be a lot or a little.

2The reason Joseph mentioned separately that it would be "to eat for your children," instead of including the children when he had said that the 80% would be "for you and your households to consume" was, that the portions required by the children would be greater than that of their parents.