My order of parks for the kids would probably be
1. Magic Kingdom: There's just so much magic and fascination for the kids at this park. By far, this park has more for the kids than anybody else. You have tons of dark rides that they can ride and enjoy such as Peter Pan, Winnie the Poo, It's a Small World, Buzz; but you also have tons of character meet and greets and just magical fun for the whole family here. There's enough thrills to make a slightly older kid happy and enough tame stuff for the younger children and as an adult you'll love them all.
2. Disney's Animal Kingdom: Kids love Animals and they are everywhere. The Safari is a great ride, Rafiki's Planet Watch has a petting zoo, there's some great shows such as the Festival of the Lion King and the upcoming Nemo show, then there's the dinosaur themed land. There's also some bumpy and thrilling rides for the older children in Expedition Everest, Dinosaur, and the River Rapids. Did I mention there's animals everywhere? If your kids have any fascination with animals they'll have a field day in this park.
3. Disney's MGM Studios: This park is a little light on the rides for children as I wouldn't recommend Tower of Terror or Rockin' Roller Coaster for them, but there are tons of characters all over the park and some fun shows. The animation studios may still be fun for the children (I don't know as I haven't been since they took the actual animators out of the park) and they'll love fantasmic to end the night.
4. Epcot: In my opinion this park is one of the better parks, but I'm an adult, to children I would imagine this park might be a little boring especially to small children. There are very few character meet and greets throughout this park and half of the park (The World Showcase) may just be boring to the kids as it's mainly just theming, shops, and restaurants with a few small shows and circle vision theatres thrown in. However Future World isn't too bad for kids. If they're old enough, there's the "thrill rides" of Test Track and Mission Space, there's also dinosaurs a plenty in Universe of Energy; a pavilion mainly for the kids in The imagination pavilion with the journey into imagination ride with figment and the honey I shrunk the audience 3d show, The land has Soarin which is lots of fun, and the kids will love Turtle talk with crush and the new nemo ride coming to The Living Seas (not to mention lots of fish) although this isn't open yet.
So all in all, imo the best park for kids (and I'm assuming around 2-8 years of age) is Magic Kingdom by a long shot followed by Animal Kingdom, and then a close tie for third with MGM and Epcot with MGM getting a slight nudge.
Also be aware that if I were ordering these parks for adults that the orders would change. Anyhow, these are just my opinions though so take them as that. I don't even have kids so what do I know.
Just one more thing, remember to have a magical day.