This past holiday season, MK was blocked out December 17th/18th (can't remember) till January 19th. The other parks are typically December 26th-30th/31st. Your DD will be able to buy tickets at a discount (not sure what the actual discount is) which is the only way you would be able to get in the park at that time.
CMs CAN get in during block out periods, however as was previously mentioned we are the first people denied entry when a park starts to go through phase closures. I believe the first phase of closure also includes people on non-hopper tickets who are staying off property if this affects you. This WILL happen during that post Christmas week, especially the 28th-31st, so be prepared!
Where is she doing her PI? I worked 56 hours during the post Christmas week with only one day off (which is fairly common) but if she's not in the parks things might be different.