Our first trip to WDW in August 2004 DD was 3 almost 4, was a big success. We did:
MK days 1 & 4
MGM day 2
DTD/rest day 3
rest/change hotels day 5
AK day 6
USFL day 7
IOA/drive home day 8.
I think MK has to be first it really psyches them up for the whole trip! Remember, even the best laid plans fall astray when dealing with young children...be willing to be flexible and follow THEIR lead. My dd loves the character encounters more than anything! She loves rides, but at WDW she prefers autographs and pictures of the characters. We went in February and May of this year and are going later this month, we now go to Chef Mickey's on our arrival day for dinner. If time allows we go to MK long enough to meet the characters right in the front before we go eat. My dd loves this plan! She is always willing to go to the other parks since our last day is always MK.
She only tolerates Epcot for 1/2 day at the time. The park is too big for her and not much interests her after we find "The Beast"

This usually takes a while though, she is willing to do anything in order to see him.
You will see Kodak picture spot signs around the parks, these are your obvious photo opportunities

I look for opportunities when dd isn't expecting pictures. Some of my favorites from our trip in May:
close up on the carousel of DD and the horses head
dd on the boat crossing the lake in Epcot
DD in France in the library at Epcot
DH holding a sleeping DD in the line for the busses at the end of our last day at MK
DD and DH in Toontown with Mickey
Our dd loves the character meals too, I do as many as our budget will allow.
We have found 10 am and after 8pm good times for Toontown characters. DD was first in Toontown in May and got a personal tour of Mickey's house from the mouse himself on his way in to the judge's tent!
DD also loves the playground at MGM even gave up AK in February to have a second day to go there. She likes all of the shows there too. We did Epcot instead of AK and MGM in May. She liked the Fantasmic show better than Wishes. She loved the Spectromagic parade too. She likes the daytime parades but says she gets too hot
I hope you find some of this helpful!
Lisa