In 1998, our family made our first trek to the World. We arrived the Friday before Thanksgiving and returned the Friday after. In a nutshell, we found the crowds very manageable. The numbers picked up ever so slightly each day of Thanksgiving week but not really noticeable until Thanksgiving Day. We arrived at the MK for EE on Turkey Day and booked out of there by noon as the crowds swelled dramatically. Spent that evening at MGM and it was pretty quiet there.
A couple of notes that may be of help. AK had opened earlier that year and not sure if it had any affect on crowds (Fantasmic! was also new that fall). Our party consisted of my wife and me and our children, ages 8 and 5 at the time.
Feel free to email or pm me for further information.
Oh yeah, just the opposite of you, we are planning on going President's Week 2004 instead of Thanksgiving 2003. Our kids will miss no school since they get the week off.
Paul