It can work if you have enough people on your
MDE to place in each room, without duplicates.
Make some way to track which MB are which, like all for each room the same color/design.
Select 5 people in YC room, get 5 MB.
Put 4 people in Riviera room, get 4 MB.
Pay for the extra adults as applicable. Easiest thing is to have 2 instances of some members in MDE, for instance, original copy of you will be listed in Riviera and a back up of you listed in YC (name the copy something like Firstname-YC).
All tickets will be assigned to the "original" copies and that's the band to use for parks, purchases, etc. The band assigned to the back up you is only for accessing the YC and it's facilities.
I've aways had back up copies of several of the members of my travel parties so we can move tickets we want to save to avoid using the tickets unintentionally. There are lots of reasons to have multiple copies of a single person.
Swap around each of the MB as needed to access what is needed. Example, you'll need the YC MB to enter that pool.
If all MB are at the pool the grandparents could still access their YC room using the MDE app on the phone, signing in to whoever is lead guest in the YC room.
Not all grandparents are old and infirm (quote from Mr. Fredrickson). They could easily just be in their 50's and have no problems at all with navigating something like Disney. Even a first time visitor.