As for your dining plan question, you can't get a partial dining plan. Everyone on your reservation must buy the dining plan for the entire length of the stay.
I can try to answer the ME question although the place for that is the transportation board:
All guests using ME must be registered on the reservation (except those under age 3). This could be a problem on a package reservation. If your son and daughter in law do not want to buy the same length of Disney theme park ticket you do, or they do not require theme park tickets, you will either have to buy everyone the same type of ticket or not list them on the reservation. If they are not listed on the reservation, they can't use ME.
One solution is to purchase the shorter length of ticket needed (the ones your son and daughter in law will need) and then upgrade your ticket to a longer one.
If you are not on a package reservation (that would mean you are using
DVC points), and you list your son and daughter in law on the reservation to get them on ME, they'd still have to buy the dining plan for the full length of the reservation.