Hi Yamylene Anne,
Thanks for your excellent question, and how awesome that you are going to share a special vacation with your grandma.
While the
reservation cancellation policy calls for a no-show penalty of $10 per person if you miss a reservation entirely or fail to cancel 24 hours in advance, I've never had to pay a penalty if one or two people in our party can't make it at the last minute. On a Thanksgiving vacation two years ago, our daughter came down with a stomach bug suddenly and needed to sit out dinner at a signature restaurant, and it was no problem.
I've also had occasions where we had reservations for large parties of 10 or more and ended up with a couple fewer than we anticipated and, again, it wasn't a problem.
If you know she's not going to be able to accompany you to dinner earlier in the day, it would be a good idea to give Disney Dining a call at (407) 939-3463 and let them know. Or, if you find at the last minute that you can't make your reservation at all because of your grandmother's health, I'd also give a call to reschedule. Cast members do their best to accommodate such requests based on the circumstances.
I hope that eases your concerns. Please let us know if you have any other questions about traveling with a multi-generational family. Many of us have experience with visiting Walt Disney World with little ones and grandparents, so we're happy to share out advice.
Enjoy your vacation!
Wilma