We went in Dec when my boys were just over 3 and 5. The older one is a thrill seeker, very outgoing and adventurous. The younger one is a little more shy & fearful. Both are high energy and have always loved carnival rides.
Fireworks - DS5 loved. DS3 was asleep. We didn't do Illuminations or Fantasmic
Spectromagic - a huge hit for both
Splash Mtn & BTM - a huge hit for DS5, DS3 was too small
5yo MK faves were Splash, BTMRR, Haunted Mansion, Pirates
3yo MK faves were WDW RR, Goofys Barnstormer, Haunted Mansion & Pirates & Indy Speedway
(neither really ever found anything too scary, except maybe in a fun way -Haunted House included)
Both boys loved meeting the Pixar characters at DHS (older DS loved all characters whereas DS generally avoided or was happy to watch big bro get his autograph). The car stunt show & Backlot Tours were faves here.
We didn't do much at EPCOT (due to lack of time) but the older one loved collecting his mask charms for the KidCOT funstops. The little guy was not interested. Nemo ride was a hit. We missed alot of the good ones
AT AK the kids loved the safari & the Nemo show. My younger son FREAKED when he got soaked on Kali Rapids and he got SOAKED. And he generally doesn't mind getting wet but had never really been on a water ride. Keep in mind there are a few sprinkler type areas in each park (like Donalds boat in Toon Town) so you may want to either avoid or be prepared. I had changes of clothes for the boys (which they didn't really need) and had watershoes to slip on before they ventured in.
Other things--the boys seemed to think any mode of transport was a ride. The buses, monorail, ferry's. So it was easy to keep them happy. --Don't forget the little things. My kids were thrilled to eat a mickey bar or get a new hat or balloon. --The pool was a huge hit
Have a great time.
Wish we were going again soon