Your main park for decorations will be MK, and it definitely warrants some time and pics - very magical for first time(every time for me!), but especially at Christmastime - you will be amazed I am sure! Are you going to MK twice during your trip? Are you going early to mid month or close to Christmas (makes a difference for crowds)? I am guessing you have FP set up, so if you are splitting it into two days, you should have 6 favorites set up which gives you breathing room as well, and with a 13 yr old, you can go all day I would imagine. If two days, I would recommend rope drop both, and can take a little time on one of them to take in Main Street. Also, since they are having everyone wait on Main St. before the Welcome Show now, if you get to park an hr. before opening, you will have time to take it in while waiting as well. The only other park that has some of the details and decor as you enter will be HS - the storefronts are cool, and will have some Christmas decor, but you can also check out later in the day. If you have time in your schedule or are heading on the monorail for a meal, be sure to check out the Grand Floridian lobby with the life size gingerbread house, which you can also buy some to taste or take home as an ornament.