Everyone in the room has to have the same dining plan. If you are booking two rooms, then one room can have the Deluxe Dining Plan and the other can have the Quick Service Plan. If you are booking only one room and the adult kids don't want TS meals, the DxDDP probably won't be cost effective because you'll be losing money if they are only doing QS.
Currently, Disney will allow dining plan credits to be used by anyone so at a TS meal the DxDDP credits can be used to pay for meals for the adult kids who are not on the DxDDP.