The two witnesses rule almost guarantees that the elders* will not convict a child abuser (unless they confess. And what are the chances of that?) Parents who press the issue are in danger of disfellowshipment for gossip, particularly if they try to warn other parents within the congregation, among other things.
* And this is one of the main sticking points: police work and investigation of crime is the purview of the superior authorities, i.e. secular governments. The various BOEs have no business in these matters.