Hello and welcome to the DIS! We will be there in March (1-8) with our 5 year old boy (and his little bro) as well.
We have been the last two years with him, at that time of year, and usually break MK up into two days. Where is your character meal?
I am sure you know that the best strategy is to avoid an EE morning if you are not staying onsite. That said, arrive before the park opens. At opening, we usually walk down Main Street and hit Fantasyland first, and can get all the rides in in about 90 minutes tops, including repeat rides if desired. (But we avoid Snow White because it is too scary for him.) There is nothing like walking onto Dumbo to start your Disney vacation! Then we might do Tomorrowland Indy Speeday which our son loves (and which gets very crowded as the day goes on), and then Buzz Lightyear, both in Tomorrowland. Depending on crowds, you might get a FP for Buzz earlier. But we have not needed to do this in early March. Then off to your lunch.
[We always eat at Crystal Palace, the character meal buffet at MK. We just love it (for what it is...)! But getting out of the park sounds like a nice idea too!]
After lunch, you might concentrate on the other side of the park. Our son loved Splash Mountain and Big Thunder Mountain Railroad. But he also loved Tom Sawyer Island and Swiss Family Treehouse. You may well need to get fastpasses for these rides right after your lunch, even during this only "moderately" crowded period. At about 3:00 (I think) it will be time for the parade and there is no need to rush over to Main Street (unless you want to) as you can get a very good view in Frontierland.
I haven't mentioned Toontown Fair. It is off Fantasyland and usually opens at 10:00. There us a great little coaster there if he is not up for the bigger one. But beyond that it is just an elaborate character meet and greet. If we had only one day at MK and had scheduled a character lunch, I would personally skip it.
Anyway, this works for us, but I am sure you will get lots of advice! Have fun!
