We have NEVER had a problem checking in and we usually check in around 11:30 a.m.. If the room isn't ready we just have them hold our luggage and go take a stroll around the grounds. I find this a GREAT time to take pictures of the Resort because we are not usually in a hurry to be anywhere ,so you can go to all places you want to and get great pictures. If we were hungry we usually grab something at Gasparilla Grill (quick food), although; we tend to start our vacation off with PS for Cinderella's Galla Feast in the 1900 Park Fare Restaurant so we starve ourselves most of the Day. They have an Excellent Buffet with Fantastic Food. We have 2 DD's so this is up our alley. Like Joanna said you may want to bolt over to MK or take a swim to refresh. Either way, you can't go wrong. Your back HOME at the Most Magical Place staying in the Most Magical Resort. We just wing it. It's all gonna depend on how you, hubby and the kids feel when you get there.
Also, if the room is ready we usually use the time for me to unpack, settle in the room and get the kids down for a nap. While my husband runs to Kissimmee to the Super Wal-mart to pick up anything we forgot. That usually only takes 1 - 1 1/2 hours so you still have most of the day. Just a side note.
Have a Great time.
Wendi