You can't get the DDP on a room you booked through a convention. This is not a Magic Your Way package reservation. To get a dining plan you will need to book a package reservation yourself, which would result in losing any discount you'd be getting on the room by booking it through the convention.
Maybe you can suggest to the parents that they provide a Disney gift card for their children to spend on meals and souvenirs. The parents can give them a specific amount of money to spend if they put it on a gift card. The downside is that if the kid loses the gift card, they lose the money too. The adults could hold the gift cards, and write on each one with a sharpie so you know which gift card is for which child.