Hi Jessi,
You'll find the crowds at Walt Disney World increase in direct proportion to when schools have breaks. March is when many spring breaks begin happening for schools, so the crowds will as well. However, it isn't Easter week yet, so it won't be as crowded as it could be.
With all that said, don't let the idea of possible crowds discourage you. We were just at WDW between Christmas Day and New Year's Eve, the busiest week at WDW. In fact, there were times when the parks were closed because they had reached maximum capacity. Through early arrivals (being there when the parks open) and use of fast-passes, we avoided long lines. We also carefully utilized touring plans through www.touringplans.com that are researched based itineraries that provide guests with the best days to visits parks and the order to visit attractions. It does have a small cost, but one that we could very worthwhile.
I'd also encourage you to make sure and have dining reservations made in advance if you plan on dining at table service restaurants. From experience, it is nice to dine at places where you don't have to stand in line waiting for food, especially with little ones.
Character dining is also amazing; characters come to you rather than standing in line to meet them.
Please let us know if you have any more questions; have a great trip!