We always celebrated half-birthdays when I was growing up. My birthday is Dec 17 and my sister's is Dec 30. This always fell during Christmas Vacation in school, plus we lived in MN and it was always cold in the winter. June was a much nicer time to have a birthday party! I thought it was a great solution - plus it avoided all of those dreaded "combination Christmas/birthday gifts".
I think kids take these sorts of things at face value - just say you're having a half-birthday party and they'll be thrilled. We had our party for the half-birthday, but our "real" birthday was still acknowledged with our family (a gift from mom and dad, and my mom would always cook us whatever we wanted for dinner, plus make a cake).