I have to admit I'm surprised by the number of people saying to do MK first. There's a reason why in show business they always say to save the best for last. Particularly with a group of first-timers, there's a risk that MK first will set expectations for the remaining parks, both in terms of rides and in terms of atmosphere, intense stimulation, and style of the park. Conversely, and particularly if they're starting the Saturday that is two days after Thanksgiving, long lines and crowds could make the experience frustrating.
If seeing the castle for the first time on vacation is magical the first day, how much more magical is it if you wait until the third or fourth day?