With
DDP, the server will first bring you a receipt showing the entire DDP bill, including tax & tip. (If you're one of those who want to know the prices of the meals, write down the total at this point because you won't see it again.)
They'll take your room key, run it, and return with a receipt which shows no prices and a total due of $.00. At the bottom of that receipt, you will see a notation of the number of TS credits remaining. I'm not positive of this, but I believe we got separate child and adult bills and receipts.
[ETA: For any non-DDP items (cocktails, for example), they will bring a separate bill. Since you are on DDP, that bill will INCLUDE the automatic 18% tip. You pay that bill with a credit card, or by room-charging, in the normal fashion.]
Because of the occasional reports of erroneous charging to rooms here on the DIS, we decline room-billing and just use credit cards for all our purchases. So I don't know how that differs from the above, but I'm sure someone will answer that for you.