Shirley,
First of all, Happy Birthday to your mom from all of us at the Moms Panel! How exciting, and especially nice to know that she got to grow up with Mickey Mouse from the very start!
I think most people, including myself, would pick the
Magic Kingdom if they can only pick one park to visit. It is the park with the most variety of offerings for all ages, and it also is where you can find Mickey Mouse fairly easily.
In general, to find characters you should pick up a "Times Guide" at guest services (and anywhere else you can pick up a park map) and look for the times and places for character "Meet and Greets." If your desired character isn't listed, you can always ask a Cast Member, and they will be able to help you by calling a "character hotline" to find out the times and locations of any character in any of the parks and resorts.
Fortunately in the case of Mickey Mouse it is easy to find him at the Magic Kingdom, since he has his own special location in Toontown. You can almost always find him over there waiting to take a picture with you at "Mickey's Country House & Judge's Tent"
Hope you guys all have a magical visit!