Hi:
We went to Disney World three years in a row starting when my two boys were 4 and 7. The park they originally liked the best was Animal Kingdom. We live in California so Magic Kingdom is like Disneyland so the newness factor was not there as we frequent Disneyland a lot. But if you are new to Disney, Magic Kingdom is a classic disney experience.
The most surprising park they loved is Epcot. This park at first looks like an adult 'land' but it is packed with activities and rides that the children love. As the adults enjoy the different countries, the children can collect stamps and passports from each country. There is usually art projects they can do also. In addition, there are rides in some of the countries like Mexico and Norway. Test Track is a great ride and so is Soarin'.
The park that I would avoid if your children are under ten is MGM. The rides are meant for an older crowd so if your children are young there is less to do. I also found this park dirty which was interesting because usually Disney seems so clean. There were cockroaches around the seats where you watch Fantasmic. Gross!
What parks and what order you go to will depend on where you are staying and what hours the different parks are open. Usually some of the parks stay open later than others so its important to go on the Disney website and find out their schedule. Are you staying in a Disney hotel? You can get Magic hours and special wristbands that also allow you to stay later. It's important to plan your stay by their hours.
Animal Kingdom generally closes and opens earlier. So depending on what time you get into Orlando and how tired you are the first morning or how excited and ready to go you are, will determine if this is a good first park. Getting to this park first thing in the morning is important because the crowds are thinner, the animals more awake and potentially it's cooler.
Get prepared for not doing everything. (It's why we went back three years in a row plus we had a blast each time). Call to arrange reservations for meals if you are eating in the parks. There is a disney dining number and the staff can help you choose the right eating experience for you.
You can also go on their website and download maps especially made for your family and the experience you want to have. Another good strategy is to put shows inbetween going on rides so you are sitting down inbetween walking everywhere.
Have a great time! I wish we were going this year!