At least a loose plan is important. Take a look at some park maps and get to know where all the attractions are. The last thing you want to do is cross the park several times getting to the attractions you want to see.
If you are going for a short trip, then planning is more important. You're traveling with a little one so you'll want to try get to the parks early and then back to the hotel for a break in the afternoon.
Lots of people swear by the "Unoffical Guide's" tour plans. We used them one year when we went with a group of 8 over spring break. It worked great!!!
When my own family goes in the off season (Nov/Dec) I've adapeted that plan a little. DH HATES to be overly planned so I have divide the MK into areas with our favorite attractions, so when we get there and find we've wandered to a side of the park we do the whole list (in order of buisiest to slowest) before heading for another section fo the park.
I would say the more familiar with the layout of each park, and what your family wants to do, the better off you will be (we always get our favorite rides in first thing so we can do them twice). A good guide book will help you there.
I like, the Unofficial Guide and Birnbaum's. I know LOTS of people have raved about the Passporter too!
Good Luck!!! You're going to have loads of fun!!!