In retrospect, others may have given signals that they did know. Growing up in Chicago, probably one fourth of my friends were Catholic who frequently made jokes about priests chasing around after young boys. I was too young to be offended, and the tone of the claims was casual and along the lines of gallows humor. Yet they were oddly prevalent. I remember wondering, "Is this just kids talking? Are those priests sickos or are my friends the sickos?"
Some of this assuredly got back to parents. Some would likely admonish sons for using sacrilegious humor. Some would probably deny it to themselves, hush it from their friends to avoid appearing disrespectful toward the Church, and of course as we now know, some parents and police officers were simply bought off by the Church not to press charges or say anything. But whatever social dynamics were involved, the Church was able to keep it hidden, even while professional child advocates were negotiating payoffs for the victims.
Even the reporters at the Boston Globe didn't know what was going on when they first started investigating. The media exposing it may have been a major break from well established journalistic tradition.