Really? They were assigned tasks as part of their job which they failed on several counts.
Got her age wrong
It's not the job of the police to be precisely about the age of every person they speak to. They don't have a right to demand ID from everyone they meet during the course of their duties.
Got the age of other underage girls they were meant to be assessing wrong
There is no suggestion of that in the report.
Missed that a specific girl they were looking for was there
It doesn't say that in the report, either.
Didn't enforce the law or safeguarding regarding underage drinking
How could they have done that? There is no suggestion that they witnessed the purchase of alcohol or the drinking, nor indeed that they would have known that his girl had been drinking.
So what did they do wrong, Graphista?