They have two special evenings with themed parades, etc. There is Mickeys Very Merry which will be on at the Magic Kingdom (like the Mickey's Not So Scary Halloween nights but Christmas themed. Hollywood Studios is running Jollywood Nights and that's a first for this year. The Star Wars land is there too.
I've not been to either but Mickey's Not So Scary is great - the park is closed to everyone except for ticket holders and there's lots of themed events, special fireworks, the characters are all dressed up in Halloween costume, etc., so I am guessing the Very Merry will be similar.
You need to book tickets for these.
The Frozen themed stuff at Epcot is amazing and there is the Coral Reef restaurant there which has the aquarium walls as part of the restaurant so you can see all the fish and the odd scuba diver. Your 6 year old in particular would probably love the Turtle Talk (I think it's called) with Crush which is a sort of interactive film.
If you want to bag the maximum number of princesses (you can get autograph books in the park stores, or take your own if you prefer, and kids love getting to meet them and get their autographs) then Cinderella's Royal Table will have loads of them going from table to table. Be My Guest will have Beauty & the Beast characters.
If you're going to Disney Springs the Boathouse is a fun themed restaurant. There's also a couple of amphibicars going around not far from it that your two may like looking at. That's where the big Disney store is but there's also a large Lego store with huge Lego sculptures that your two may like looking at.
You do need to book the parks (you can change them later if you want and there's availability) and the restaurants as they get booked up. You're best off deciding what parks you want to do on what day now and working from there. There's also a Disney app (can't remember the name of it) which tells you which park is quietest when.
There are some very good Disney forums around so it may be worth you having a look at them and getting some advice from the regulars.
Happy Holiday!