Hi Alice. Your question makes me so happy. We have traveled to Walt Disney World Resort several times with both my parents and my in-laws and they are similar in age to your parents.
Their favorite thing to do is to watch their grand babies stare in awe of all that is Disney.
Your parents will have more than enough to keep them busy and very entertained. The dining options are fantastic throughout Walt Disney World Resort. They might also love to stroll through Disney Springs where they can enjoy shopping, dining and entertainment. The spas are fabulous or golf is another great option. They can also enjoy Disney's Boardwalk where they will find shopping, dining and street performers.
There is plenty for them to do while in the theme parks as well. A can't miss experience with little ones is meeting Disney Characters. They will be filled with Disney spirit when they see their grand babies interacting with Mickey, Minnie and the gang.
The parades are fabulous and there are numerous shows that are can't miss as well. My daughter and parents thoroughly enjoyed watching Finding Nemo - The Musical together. Don't forget about the fireworks!! They are the perfect way to end your day together.
If they need more to do, there are several amazing tours they can go on.
Disney's Keys to the Kingdom Tour at Magic Kingdom and
Behind the Seeds at Epcot are two of my favorites.
For the majority of our vacation, our parents stayed with us and sat, relaxed and took in the sights if we were enjoying an attraction that wasn't a good fit for them. They loved it and I know your parents will too. Take LOTS of pictures!!! You will be making countless magical memories.