I was dropping off my parents at MCO
about 3 hours ago and it was pretty empty for the holiday season. Although it was somewhat full compared to off season days. If I were you, I would tell your parents to leave St. Pete about 5 to 6 hours in advance, especially on Christmas Eve Day, to the airport because of the traffic on the way and then you have the delays because of passenger hold ups in the airport.
Merry Christmas and Happy Holidays!!