Just got back with a 3 & 4 year old and I too had planned what day, where, and had all ADR's planned. Other than that I just had a list of things I definitely wanted to hit. I think you will manage fine without planning what and where. We found it to be busier that expected and just bypassed rides with long lines and utilized fastpass when able to - esp. when me or dh wanted to do big attraction rides the kids wouldn't be able to ride. Luckily our children took the response "we can wait here for 2 Dora's or we can ride something else and come back later and do this" as rides like Dumbo can get quite backed up. I would plan on hitting fantasyland 1st - as soon as you get to the parks as those rides are quite busy by midafternoon but take things in any order as you see fit with your 2 young ones. Another recommendation if you haven't already thought of it but plan on either bringing or renting a double stroller as both will want to get off their feet.
Big rides for my DD3 who was a little more intimadated by rides were: The teacups, dumbo, aladdins magic carpet ride, the playgrounds (1 in MGM and Poohbears playful spot in MK), the race cars the kids can steer around the course (you have to press on the gas pedal) and the carousel. We did these rides more times than I could count. They loved the country bears show and the tiki birds too (although I found tiki to be loud). My daughter chose this trip to show us she could decide what she did or did not want to do so we were unable to get her onto/into some rides/shows and in particular rides that she could not see what was going to happen. If it was out in the open she was fine, but if the ride went behind doors and she couldn't see around the corner she didn't trust it so it is good to go with a mindset that you might no do too many rides - you really have to take it at their pace and attitude. I had planned on both kids doing so many more but they still got alot out of this trip so have fun with it. My son was a little daredevil and at 4 did Big Thunder, Haunted Mansion, and more so it will definitely depend on the child. Other things they really got into: pin trading (their grandparents started them when the pins first came out so they already have quite a collection), autograph books - tons of photo ops, epcot kid stop spots - we did both the masks (free) and purchased the passports (10.00 each). If you do the passports have a phrase in mind (have a nice day, welcome to my country, etc.) for all the CM's to write so that looking back you will know what they wrote in Chinese, French, etc. But relax and...
Have a GREAT trip!