I was at WDW this past March: 17th (St. Patrick's Day) thru 20th (First day of Spring), and found the parks to be somewhat crowded. Epcot on Friday the 17th was packed during evening EMH, to the point that I gave up and went to my resort. And the Animal Kingdom was crowded because Expedition Everest had just opened.
There were groups of children all over the place (teams, cheerleaders, etc) and while I enjoyed chatting with some of them, being a retired teacher myself, they did make for some crowded spots at times.
Also adding to the crowd are the people who come to the Sports Complex to see the Atlanta Braves baseball play spring training games. (That's why I made the trip myself.)
March is much better crowd wise than the hot summer months, but not as good as the period between Thanksgiving and Christmas, in my humble opinion.
Enjoy.