I have the opinion (not at all based on any sort of facts, just observation) that people fall into two camps: they either love Disney, or hate it. I wasn’t sure which I would be, but it turns out I love it. No judgment for those who don’t — we all like different things!
We took our kids for the first time when they were younger than yours but still old enough to remember the experience. They LOVED it. They loved meeting the princesses and pirates and other characters and, like Memphis Blues mentioned, they thought it was magical and that all the characters were real. We did the makeover thing, with costumes bought on sale at home and saved for the occasion (they have makeovers for boys and girls both now!) and it was really cute to see all the little princesses and pirates running around.
And I second the vote to stay on the resort if you can. It makes all the things easier if you can just get up, get dressed and get on the bus and go to the park. We live within driving distance of Disney World and have now been multiple times. Park employees and Orlando residents have told me that September and February are the lowest times for attendance and park crowding. We go on the off season — November or early March — because we find that the heat in the late spring and summer makes standing in lines unbearable.
My only other tip is: take in a backpack with lunch (I find the food to be not very good and expensive and I eat better if I take in food) and a book or two (for long lines and a break from the crowds). Pace yourself and don’t try to do all the things. Prioritize and use Fastpass and enjoy the experience.