Tammy,
If this is your first trip to Walt Disney World, I would recommend doing the Magic Kingdom. There will be rides/shows/adventures for both of your children in this park. The Magic Kingdom is the "core" of Walt Disney World in my opinion.
I am not a "certifiable" Disney addict. I commend you for taking the time to read this board. I did the same for my last Disney trip three years ago. I found a WEALTH of information. Some of it made my trip easier, some was probably good for the people that posted it, just not me. You will have to decide what is most important to you.
Here are some things that stick out in my mind that you might want to consider:
1) If you schedule a character breakfast on a day you plan to visit the park, it will take away a considerable amount of your park time. Unless, of course, you have this breakfast very very early. We did Cinderella's Royal Table and had a relatively early time. We found that we were disappointed in the loss of park time because the lines were longer by the time we finished breakfast. Of course, our children were a little older than yours, and that could have played into that feeling also.
2) I read in a book this tip - Go to the Magic Kingdom and be there when the gates open. When you go in, take the train to the back side of the park and work your way forward. I did this when I went with my daughter several years ago and we got to ride many rides over and over before the crowds made it that far. It was VERY nice.
3) We enjoyed the character breakfast at the Contemporary resort and we also enjoyed the monorail ride. We got there early and were able to ride in the front of the monorail. My daughter LOVED it (she was 5 1/2 at the time). When we did this, we didn't have a park day scheduled so we could relax, ride the monorail, have our breakfast with the characters and take our time.
4) Get a stroller!!!! We pushed our 9 year olds through the parks in a stroller. I know that sounds weird, however, it made it a lot nicer than hearing "my feet hurt" all the time. We also had a place to hang backpacks, etc.
5) Remember you need to make a "priority seating" if you plan to have a character breakfast inside Disney. My first trip there, I didn't realize that. I was truly naive.
My last comment is to ask all the questions you like. You might think they sound "silly", however, none of them are. I asked lots and lots of questions on these boards and got many many answers.
Good Luck and Happy Trip Planning!!!