1: For your son and GF to be on the dining plan, they have to be listed as guests staying with you in the resort (you mentioned they would be joining you "at the parks," I don't know if that also means "in the room").
2: But that means you'd either have to list them as guests staying with you the entire time, paying for those extra credits when they're not with you, ORRRRR
3: Have WDW essentially split up your reservation into three smaller reservations - the days before they're with you, the days they're with you, and the days after they go. Theoretically, this could involve switching rooms at least once (I've never heard of this happening but when people split up their reservations like this they're always warned that they might have to move but Disney will make every effort to avoid it).
4: Regardless of whether you list them as with you for the entire trip or part of the trip, if your son and his GF are over the age of 18, you might have to pay extra. Hotel room rates are usually based on 2 adults occupying the room maximum (plus kids). I don't know if it applies to adult children as I don't have adult children, but bear in mind you might be paying extra for the room as a result.
I honestly think the simplest thing to do is this - buy a Disney Gift card and put $100 for each day they're joining you. Call it a "meal allowance," let them pay the balance out of pocket. You're still helping out, this affords them a little more flexibility (example, the QS dining plan isn't worth using for breakfast but maybe they'd prefer a breakfast and a late lunch, this will let them maximize that "meal allowance"). If they're hurting for a refillable mug, buy them one if you want, let your conscience be your guide. But this seems to be a better solution over making multiple reservation changes.