We will be staying on site and my parents will be off site.
What would be the best way to plan our FP for all of us to go together on any given ride?
Is that even a possibility?
You would need to add them to your Family & Friends list first. If they have their own MDE account, you would need to use the 'Invite to Plan & Share' function to connect with their MDE account. If they use a single MDE account for both of them, you would need to use the 'Invite Through a Mutual Friend' option to add the other after you've added the primary account holder. If they don't have (and don't want) their own MDE account, you would add them as manage profiles (you would manage everything for them).
Since you are staying on-site, you can make FP+ selections at 60 days from check-in if you have park admission linked to MDE. If you make your selections then, you can then copy your selections to them (subject to availability) once their FP+ window opens at 30 days from day of use.