Hey There, Hi There, Ho There, Meilin!
Welcome to planDisney! We are THRILLED you stopped by and are honored to help you plan your upcoming trip to 
Disney World! You are definitely visiting during the most WONDERFUL time of the year! It also means that decorations will be everywhere, Christmas tunes will be in the air, and the scent of holiday treats will be around every corner. OH, and there is nothing like experiencing the MAGIC of Christmas with your family! (You can also tell that you are an expert-level planner since you plan to take your little ones to nap during the afternoon.)
Now, since you will only be visiting 
Magic Kingdom during the day on a 
Mickey's Very Merry Christmas Party night, the park typically closes to day guests at 6 p.m., which means you can enjoy the holiday enchantment right up until then. Technically, that would mean you could enter the park up until 5:59 p.m., but at that point, attractions will already be closing to day guests, and you would be able to soak up a few moments on Main Street U.S.A. before leaving again. 
If you arrive back at Magic Kingdom around 3:30 p.m., you would then have a solid couple of hours to squeeze in a few more attractions, grab a Dole Whip at 
Aloha Isle, or enjoy a castle show before the MAGIC shifts to the nighttime crowd. Keep in mind that some areas may begin closing early as Cast Members set up for the party, but you’ll still be able to enjoy that cozy holiday feeling as the sun sets over 
Cinderella Castle. You really do have the perfect plan to enjoy the park when it’s merry and bright, recharge during nap time, and return refreshed to make more memories before heading out. Even a half-day at Magic Kingdom during the holidays feels like a full day of joy — and that’s the real Disney MAGIC!
If you have any other questions, please don't hesitate to ask. We are here to help in any way!
Have a MAGICAL day!
Wes