It's hard to tell

Legally, officers are obligated to prosecute all the crimes they are aware of. But in real life, not all crimes, even of the same type, are really the same, and not all perpetrators are equally bad. An officer with some experience, decency and common sense probably can realize that, and may even be able to act different in each case. Of course, such cops seem to be very few. And regarding child sexual crimes, even fewer.