I was able to do this recently, changing a reservation from 4 people to 3. I'd encourage grandparents to go. It can be a lot of fun. If breakfast seems like a waste of money (I'm not usually a big breakfast eater), try the character lunches or dinners.
If you're all new to character meals.... at first, I thought these were just for children. But we (adults) have had some great fun at character meals. My DS6's grandparents thought it was a little corny but they learned to have some fun with the characters too. It's easy to provoke many of the characters into some funny interactions by saying things to them. I've seen my wife laugh just as hysterically at the characters as our young son.
At Crystal Palace, we have kidded Pooh about eating too much of the buffet and complained that we can't find any honey...... asked Tigger if he was jumping all the time because he had too much coffee....at Garden Grill, asked Pluto if he wanted some of our food instead of the lousy dog food he gets..... thanked farmer Mickey for growing the delicious vegetables.... at Ohana, Mickey taught our son the shaka surfer sign.... silly stuff like that....