We either take something if its only for a night or two, or for when we've stayed a week we give $50, and take them out for dinner.

When we have house guests for more than 2 nights, we definitely hope (hmm, expect... but hope is a good word LOL) for a bit of help especially if we are cooking for them. I wouldn't think it was rude if you asked them, if they didn't offer help. It's hard enough to afford groceries at the best of times, let alone for whole other family!

OMG, as if you'd complain about your friends kitchen!!!!! GEEZ!

If they don't offer anything just say as they are going out the door "oh would you mind grabbing a few things while you're out, I've written down a little list, thanks" pass over the note and then quickly hurry off after the kids lol