My son was 3 last fall for his first trip, we're going back this year for Halloween. What surprised me last year was how much he loved meeting characters. We did
MVMCP and he wanted to meet as many characters as he could. The lines for characters are long at the parties, so honestly I wouldn't waste your money on party tickets. Just incorporate character visits during your park days and do 1-2 character meals. We loved Crystal Palace at MK (Pooh and friends) and Garden Grill at Epcot (Mickey and friends). I've also heard good things about Tusker House at AK (Mickey and friends) or Hollywood & Vine at HS if they like Disney Jr. characters (although the characters at that one are from more girl-focused shows).
My son was not interested in shows, only rides and character meets. Although he also had fun doing simple things like playing in the spray area and watching the trains at Epcot or the Splash Mountain play area at MK. Most of the rides at MK will be good for them and you can absolutely spend 2 of your days there. Rather than MVMCP, I would do the new Toy Story Land EMM for guaranteed rides on all 3 TSL rides and meets with Buzz, Woody and Jessie (we are doing this for our next trip).
We find AK to be a half day park due to limited activities for preschoolers. We will do Na'vi, Kilimanjaro and Triceratops Spin, meet characters, and either the Lion King or Nemo show. The rest doesn't appeal to him right now and I'm not going to risk scaring him on Dinosaur (the MK roller coasters aren't scary, but Dinosaur has some potentially scary theming).
We plan our days around nap time, and even if they are no longer napping I'd plan a mid-day return to the room to rest and recharge. We do rope drop through lunch, go to the room for a few hours, and return to the parks from about 4:30-close.