The police were looking for a specific underage girl. So I can fully believe they employed a form of selective deaf/blindness about all the other underage kids they came across on that particular, and every other, shift.
The sentencing statement says they were looking for the particular girl and others that were too young to be out, so not solely the particular 12 year old.
I do accept that it's possible that the police did suspect the victim was very young, but decided not to do anything about it, but in the absence of evidence to the contrary, I don't like to presume that their statements are false, particularly when making a legal analysis of the sentencing decision.
Realistically (and for better or worse), a judge in a criminal matter is going to rightly attach significant weight to the evidence of two police officers, particularly when it's undisputed in court, and there's a degree of corroborating evidence.
We don't have the evidence in front of us to make an assessment of its strength, all we have in terms of actual material to scrutinize is the facts established by the court and whether the sentencing statement properly applies the law to those facts. I believe it does, but that's open for debate (and that's what I'd understood the OP in this thread to be asking).