I don't think cabin crew will use their own money to buy you a gift. If I have not mistaken, from what I know, if SL or above members fly on their bday, they will either get an one-class complimentary upgrade or a box of chocolate from CX/KA.
Last time when I flew KA in J on my bday, the cabin crew got me a box of chocolate, same as what you got on your KA flight, and made me a "special dessert" by using some bananas, some chocolates and a Häagen Dazs ice cream. It's quite warm-hearted honestly and it's something extra the company didn't require them doing so...
