You have a reason to go there, which is socialising with the parents presumably. If you can't have fun with people without drinking and then blame it on the lack of alcohol... that's no good.
You can socialize and you kind of have to, but they've probably invited every classmate from preschool or whatever, so there's 20 people you probably haven't met before, have nothing in common except having kids the same age, and will probably never see again (except maybe at the next birthday party). And often the parties are held in a McDonalds or something, so catering is chicken nuggets and shitty cake with a Paw Patrol motif.
God, what a great childhood I had...
I can't even imagine a birthday party being held in some joint or something.
In my country a kid having a birthday always meant homemade food and fun (up to the age of 15 or something).
The kids are friends: that's why the parents are there. The parents may or may not particularly vibe with each other.
And a small amount of alcohol does take the sharp edges off social interactions with someone who might otherwise annoy you.
>> Democritus says: “One man means as much to me as a multitude, and a multitude only as much as one man.”