The Watchtower organization of Jehovah's Witnesses prohibits the celebration of 'secular' festivals including Christmas and birthdays:
http://www.jwfacts.com/watchtower/celebrations.php
I'm sure PG was even more fanatical about this.... So this could likely be the first birthday she's had in some time, and the first ever Christmas and birthdays the girls have celebrated. That is, IF they would be comfortable doing so... they may still consider this something sinful.
I'm VERY interested, when the time comes, to hear about JC's faith journey through all this...
EDT: Another thread, discussing JW non-celebration of birthdays, etc:
http://www.jehovahs-witness.net/watchtower/beliefs/106913/1/Birthdays-banned-in-1951