I actually think birthdays are quite important in this respect and missing out on them has a more profound effect than you would at first think to the jw youth. The WT teaches us that they glorify the individual but actually the opposite is true. When kids celebrate someone else's birthday, they learn it is someone else's special day, that they have to take a back seat and wait their turn, whilst the attention and gifts go to the birthday boy/girl. They have to give something maybe they would have wanted themselves. But then with patience their own special day comes along.
At these and other celebrations you have to think about others, There is also chance to play at party games which involve sharing, taking turns and losing plus just being part of the wider community. It may sound simplistic, but looking back at my older kids who we raised as JWs until mid teens when they have left (one a year before me and one just after me) I can see that they really missed out on a lot of life lessons through not having a part in such celebrations.