You get free express pass by staying at Hard Rock, so make sure to check in before you go to the park so you can use it. The Hagrid coaster and Pteranodon Flyer do not accept express. I think the flyers passes out return times, similar to the old fp system at Disney.
You don’t really need to plan much since you’ll have express. As PP said, just look everything up ahead of time so that you know what you want to do, and try not to backtrack since you have only one day. The Hogwarts Express is different in each direction. You are allowed to bring in as much food and drink as you like, just no glass. Many rides (some of which a six year old will be too short for) require you to use a locker. You open them with your ticket at most locations, so the max number of lockers you can get for free will be two. Free lockers are only about four inches tall, but quite wide and deep. Bigger lockers are $2 per ride. Pack your stuff accordingly.
There’s a really nice Curious George water playground that your child will probably like, if it’s warm enough. There are lots of characters to meet too, all the info is online.
The Christmas stuff starts the day before you get there. All that info is also online. I highly recommend the Grinchmas show. It’s a Broadway-quality show. It’s best to see the first performance as it’s very popular and sometimes you have to wait through an entire show to get in, but not as many people show up for the first performance. It will still be full but you should be fine showing up about thirty-forty minutes ahead of time. The holiday parade is very good too.
