We took our son at 18 months, 2.5, 3.5, and he will be 4.5 on our next trip this Oct. I was 12 weeks pregnant with our daughter in Sept. 05, then she made her first official trip to the world at last November at age 7 months. She will be 18 months on our trip this Oct. I have enjoyed every age at WDW with our kids. One tip I do have is to show them as much Disney movies/character stuff before your trip to get them familiar with the characters. They will enjoy meeting them in person much more! Unlike most people with children these ages, we hardly ever go back to the resort mid day for naps. Of course we have usually traveled during the off season when it wasn't as hot, but our kids have always napped in the strollers well. We also use the Child Care Centers often just to sit in the A/C and take a break. When you travel in the off season like we normally do, the park hours are much shorter and we don't want to waste too much time traveling back and forth to our resort. But some kids don't nap well in strollers, so I guess we're lucky in that respect. The other tip I have to offer is always go at the children's pace. Don't expect to go commando in the parks with young ones. You will not be able to maintain that pace and keep everyone in a pleasant mood.
Have fun!!!!