We were there last June and we had our 5yo b/g twins with us and boy the number of characters we saw and the time they got to spend with them was awesome. It may be that because there is an older crowd at UO and IOA that most people are not there to see the characters. We also did the breakfast at IOA and it was awesome. My son learned a butt shaking dance from the Cat in the Hat and still does it to this day. Below is a list of the characters we met.
The entire Simpson Family
Bullwinkle
Doc from Back to the Future
Scooby and Shaggy
Curious George and the man in the Yellow Hat
Woody Woodpecker and his Girlfriend (not sure of her name)
Betty Boop
All the Superheroes
Shrek and Donkey
Lucy
Marilyn Monroe (great with kids)
Jimmy Neutron
Spongebob
Dora
Barney
Baby Bop
BJ
The Lion and Penguins from Madagascar
All the Suess Characters (Cat, Grinch, Sam, Guy, Thing 1 and Thing 2)
The entire cast from Sinbad (storm came and could not do show, so they did a meet and greet and we got to meet all of them)
There are others, I am sure. But, that is the bulk of them. Be sure to check out the fountain in IOA. It was a blast. We have priceless video of my kids interacting with the fountain and both left soaking wet. Have an awesome trip.