Checking in on the 18th means you are going to be there for Christmas week. If you can get to the parks on the 18th, that will be the least crowded day. After that, I think the crowds will start to pour in. With Christmas on Friday this year, most schools will have a full two weeks of vacation, so both Christmas week and the week after Christmas are going to be crazy crowded.
But you can still manage. MK will open at 7 am every day, probably starting on Monday the 21st, so if you can get your crew up and be there for that early opening on your MK days, you'll get a ton done before 10 am. Those early mornings at all the parks will be your best bet to beating the crowds.