FWIW, Touring Plans has March 22nd-29th with resort wide crowd levels between 7-9 (7s the first 3 or 4 days, then a few nines, then an 8).
April 12-19th's predicted range is 4 (1 day) - 6 (most days).
We went in mid-march in 2013 and we actually found it was a lot colder than we expected. We actually had to bundle DD up a lot on that trip (she was 8 months old at the time).
I would think April, weather wise, would be a nice compromise between March (which can be dicey - cold or warm) and May/June (which just get hot).
With a kid in K, and it being April (not like the first few weeks of school or something), if you are comfortable with it there is nothing wrong with taking her out of school. Just be aware of your school's absence policies, whether or not she can make up any work or if she can substitute maybe a show and tell thing or something, that kind of thing. Talk to the teachers before your trip just so you know what to expect upon return (not asking permission, just so you know what's happening).
The only thing that would make me rethink that is if it is a testing week for the school..I know they have started to do standardized testing with K in some places, not sure about in yours. Making up standardized testing is a pain, so if it were a testing week I'd likely choose not to miss it, but otherwise as long as you're informed and OK with the absence policy, there's no reason not to go then.