Trusted non-kin members of society such as teachers and priests, have had free access to children.  The public is more wary though recent lawsuits and court cases but still needs to keep vigilant over their all children's associates.

Parents are often not aware their child is associating with a pedophile because the plan is so secretive and subtle.  Professional pedophiles groom multiple children at the same time. They have various contingencies in case a child moves away, or turns out not to meet all their requirements.  There are others in various stages of training at all times.  They bottom line is, a pedophile must groom replacements for children as they mature and their bodies develop.

For a child, the ability to reason has simply not been developed. Kids are vulnerable.  They like to experiment with keeping secrets.  The pedophile is a pro at finding the triggers which keep a child silent.

Children live for puppies and candy and promises of fun, permissiveness and doing anything their parents won't allow them to do.  This vulnerability gives the pedophile great influence over their chosen victim.