Not to be a dooms-dayer, but isn't May deer fly season? Just make sure they take a set of long sleeves and pants in case the flies bother them. And the bug repellant.
Also, a poncho in case you get an afternoon shower. We have been at that same time too and love it!
I have never camped at Disney in May, but we been several times during May and I must say it is my favorite month by far. Weather is great, crowds are not bad....perfect! We are camping this year in December.