I think the video was not about eating a piece of Birthday cake, but Caleb's fear of not standing up to whether he celebrates birthdays or not when his friends ask him.
I find it interesting that the the whole video does not explain WHY they don't celebrate birthdays in the first place. Simply that the bible says so and that the dad will help the son explain it. But where and what reasoning is applied? nothing.
Ridiculous.
The chairman of the Annual meeting announced that the second part of the video will be released where the father explains Caleb why JW's don't celebrate birthdays.