Disney says specifically that you are not to use your plan to buy meals for persons who are not on your plan. Technically if your parents don't order anything and eat off your plates, that's not the way Disney asks for the plan to be used, but offering a couple bites of your appetizers is likely not what Disney had in mind and should not be a problem.
And yes, in general, they will be able to share their own credits between themselves. If the signature restaurant you have in mind is CRT, or a dinner show, then no, because those are fixed price and everybody pays per person.