I am puzzled because you mentioned the dining plan and different ticket lengths.
Everybody on the package has to have the same length tickets so if you have 4 day tickets, but your grandparents only have 1 day tickets you are not all on the same reservation. If they are staying in your room their tickets/dining plan would be the same as yours.
Are your grandparents staying in a different room with a separate reservation number from yours? If so, they could have a different length ticket.
If you booked just tickets for them (no room or dining plan) and they are joining you just for the day, did you arrange to have the tickets at will call? If their tickets are NOT on a KTTW card you will need to add the additional day before the end of the MK day. I would not wait until after you return to the WL because the concierge's desk may not be open. It's hours are not as long as Guest Relations/ticket booths at the parks so better to add the day at some point while you're at the MK.