They can only df you if you celebrated a "religious" holiday.
So celebrating birthdays is not a problem as it is not religious.
By the way according to Job 1:4 Jobs sons celebrated birthdays. That really a birthday is meant is confirmed by Job 3:1 where the same hebrew word is used.
And good luck. I hope you can succeed to teach your children critical thinking skills that they will be able to make their own decisions concerning religion when they are old enough.