I think they have the checks all wrong for people who work with kids. They do these police checks to find out if they have history of a crime, but they fail to take into account the ones who haven't done anything yet or who have just not been caught.
What they really need to do is lock up anyone who wants to be around kids for a period of about 5 years, then gradually introduce them children to them and judge their reaction. You start of with a few ugly orphan kids, who are expendible, just incase of "accidents", then move onto more attractive ones. If they get through the five years without incident they can then become teachers, school caretakers, scout masters, parents, etc...