It is not "cheating" to share your meal with someone who is not booked in your room, does not have the DDP, and does not have a big appetite. Good grief. Are they really going to police each table to see who takes a bite of someone else's dessert. Give me a break. DH likes everyone at the table to order a different entree and different dessert so we all get to taste and share and enjoy everything. It may not be the way others eat, but it's the way we eat. And, we don't expect to be prohibited from doing that by silly and archaic "rules". If my fork can never take something off DH's plate, I'll quit dining out.
If I were the OP, I would share with my parents all I want. I agree that the restaurant won't redeem a meal credit to pay for a meal for someone not on the plan, but there is no way they can stop my grandma from buying a soda and a salad and then having a few bites of my pasta. Sheesh! The only "rule" I would follow is that I would be extra generous with my tipping if grandma only orders a salad and drink. I'd tip as if she ordered more.
I say the pool police on these boards team up with the dining plan police, create their own message board about following the letter of the law, do a group read of Les Miserable (in case they don't know a classic novel about the outcome of following the absolute letter of the law to the misery of all) and leave the rest of us to enjoy our vacations!