All credits are available for use from any card, so your DMIL shouldn't have any trouble using your DS's card to get him a meal. We are often split up in the morning--kids and I go to the parks while dh sleeps in. It became kind of a game to us, figuring out when dh got up ('cause he'd usually use a CS for breakfast). We'd take a mid-morning break and grab a CS to share, and if there was one credit missing we'd say "looks like daddy's up and had breakfast already!"
Just have her keep the receipts and check regularly to make sure everything is charged the way it's supposed to be. Once in a while a CM will accidentally charge to the room instead of using credits, or charge more credits than were actually used. It's not a big deal, and easy to fix, as long as you catch it quickly and keep the receipts.
Good luck, and have fun!