You have to buy a package to get the DDP--free or purchased. Everyone in your room needs to be on the same package. The DDP must be for your entire length of the reservation.
The only way you can have a room that qualifies for paid DDP without buying at least a one day park ticket as part of the package is if you are staying as DVC on points (either owner or renter).
I supposed you can have someone staying in your room who is not registered--this is probably not cool. Of course, that person won't have a KTTW card, and won't be eligible for any EMH or other goodies that need the card.
You can certainly buy food off the kid's meal for your 11 year old. He would pay full adult price at family style and buffets, or course. When he orders a kid portion off the kid menu, he would pay the kid price. If he orders an adult portion off the kid menu (double or triple serving size), he would be charged as an adult. I have ordered a kid menu meal for myself and paid kid price, so I know this is absolutely correct.