Believe or not, it's not any worse than Time Square in NYC, I believe they have security during business hours, it's right by ritzy hotels and a huge Macy's, it gets really "Christmassy" this time of year... 
Union Square - SF
In any case, why would a bunch of kids want to sing in a park where the sidewalks are covered with human feces? I can't imagine Union Square in S.F. being much of a "holiday spirit" kind of place under any circumstances.
Believe or not, it's not any worse than Time Square in NYC, I believe they have security during business hours, it's right by ritzy hotels and a huge Macy's, it gets really "Christmassy" this time of year...
Union Square - SF