G.B.
I would invite all the class, well, I am assuming that all the family are not totally girls and that some of the relatives would be boys. So the male classmates would feel okay with being there.
I attended several birthday parties when K was in the 3 and 4 yr. old programs at Head Start and they invited the entire class. It was a lot of fun and the biggest thing that I found was that the parents had just been waiting for the opportunity to meet the other parents and were so glad to have been invited.