The credits will be tied to the room where they are assigned. Only a MB from that room will be allowed to "pay."
As to the DxDDP, yes all the credits are pooled. However, the brochure AND the
free dining details both still state that kid's must order from the kid's menu. That is not how it has been implemented, but the wording is still there, so they could, at any time, not allow you to use a kid's credit for an adult meal.