Just to put things in a broader perspective, I've stayed at Chaa Creek many times over the past 12 or 14 years and have always enjoyed my stays immensely. Mick and Lucy do a wonderful job.
As silkpainter notes, April and May are usually the heart of "dry season" months in Belize and the hottest months of the year (though weather patterns have been screwy of late and you never know).
I personally prefer late summer in Cayo, when everything is green and lush and occasionally cooled by seasonal rains, or in the winter when it's often cool enough at night for a blanket.
When it's really hot, the Pine Ridge is always an alternative. The 3,000 ft. elevations are considerably cooler and less humid.
--Lan Sluder www.belizefirst.com