A tricky one. It's difficult to frame a law that criminalises this without casting a very wide net over huge numbers of people who know that crimes are committed and don't report them. Unless we make a special exception for child sex crimes as we do for some terrorist offences, which isn't very easy to justify. 😐
I think the fact that the kid was four makes this particular case rather clear-cut. If, say, the boy had been 14 and previously sexually active anyway I suppose one might say that if he wasn't terribly worried about it all then good on the parents for getting his university tuition fees paid rather than causing a fuss. But it's pretty grim. 🙁