If your ds has already mastered the kindergarten skills, I wouldn't purchase the K curriculum from Calvert - at least the first half of the math is pre-K stuff. Yawn, boring. I had to skip most of it. I'm not impressed with the reading, either - the stories are fine, as are the activities that go along with them, but your ds is not going to learn how to read with it, IMHO. I just started my ds (5.5 yo) with Reading Reflex, and love it. I'm not "sorry" I bought Calvert, b/c I needed the support of a spoon fed curriculum at the beginning of the year, but it really isn't worth the money I spent on it - I'm basically ignoring most of it! The science is so simplistic and boring, too, btw. ugh.
No, Calvert isn't Christian, but it does include "national" holidays - Christmas included.
I wouldn't call Calvert a 'classical education'.