On the visitors' front: I think that if you're going to see a couple with a new baby over a mealtime it's polite to offer to bring food for you all so they don't have to go to extra effort to feed you. Bringing food for them to consume when you're not there, though, is going above and beyond and not to be expected!
You do see advice on MN all the time to tell visitors that rather than hold the baby they can do the hoovering. I personally think this is bonkers rude and can't imagine people doing it real life, but it's a really common suggestion so apparently someone finds it reasonable.
Btw I think it's completely different if it's a single parent - I would be much more eager to offer them food, cleaning, etc. If it's a couple I don't see why I should basically do the work for the DH.