I can't comment on the park crowds for March 2, but I can't imagine they would be as bad as later in the month. As far as Regal Sun is concerned, it is the closest hotel to DtD and is definately within 5-10mins. walk, depending on how old your kids are. You could take a bus from there to any resort, but I would suggest the contemporary or Grand Floridian (might be the same bus even) instead of WL.
By the time you get to WL and then get a boat or bus to MK, you could be 1-1.5 hrs of transit time. It could be shorter if you are lucky to catch everything right away, but that's really lucky. With the CR you can just walk over to MK, about the same distance as regal sun to DtD 5mins at a reasonable adult pace. With GF, the monorail goes directly to MK and is the shortest route of all the monorail resorts.