Yeah it is just "the done thing" between our group of friends / family.
Whoever is hosting supplies beer and wine and if you want something else its BYO.
We have spirits at our house as well so people generally just help themselves. We also enjoy mixing our cocktails.
At both of our sons birthdays so far there has been alcohol supplied by us and both sides of the family and close friends have enjoyed it.
No friends from kindy / school just family / close friends.
If i had school / kindy friends I would tone down the drinking/alcohol.