The problem comes in when you have a card that says 2 adults, 2 children and you are trying to use credits for 6 people (or whatever). Technically, you have paid for the credits and it is not a problem. However, in reality it may or may not work depending on the CM involved and what they think the rules are.
The problem in treating a family you don't know is that you really can't hand them your KTTW card (it is also a charge card and may have park admission on it.) You can certainly offer to pay for their dinner and give the server your KTTW. At CS, we've had no problem buying lunch for the person in line behind us (we talk to them while in the line and order it for them).
If you know the people, you can do it fairly easily--just hand one KTTW to the server for your meals and another card for the other meals. Yes, they are all coming out of the same credit pool, but it seems to go smoother with the servers.