> A clean CRB/DBS check does NOT mean the person is not a ne'er do well.
And whether it is or not is a rubbish predictor of how good or bad they might be at working with children. I can think of a couple excellent parents and educators I know of with criminal records the size of a small library. Their experience¹ is something that children can learn from too.
¹ Well, some bits of their experience. Preferably not the parts about dodgy cheques or fake driving licences; although then again, you never know when that might come in handy.