I'm a DBS verifier and things have changed a lot in recent years. It used to be in my settings that we CRBed (the old system) everyone who had child contact, but under the new system we are advised by DBS that unless a worker will have unsupervised access to children/adults at risk they should not have a DBS check. The view was that too many people were being checked unnecessarily, and usually because organisations were scared to be not seen to do the right thing, so did belt and braces.
In this case the supervisor has an enhanced check and is responsible for ensuring that guidelines are kept to.
However, what my setting, and many others, do is good practice around recruitment. This means that a job description for every post is drawn up (even if it's a voluntary role) and the description is risk assessed around safeguarding etc. Then the applicant completes a job application form, in which they give details of previous experience relative to the post, and more generally. They are then asked to self-declare any relevant convictions, and warned that the "Rehabilitation of Offenders Act" doesn't apply. They also give two references for people who have known them in a work related capacity, or has a position to give a character reference (e.g. teacher, magistrate, doctor, church minister, not friend or relative) and these are then taken up. The referees are asked if they know of any reasons why the person should not work with C/AAR.
Workers should then be given guidelines and regular training about appropriate behaviour around service users, good practice, when to report etc.
It's not perfect, and a determined person will find a way, but it's intended to make sure that good, skilled people work with C/AaR, and to send a signal to the predatory that this organisation is on the ball and knows what it's doing around safeguarding.
What makes me cross is when volunteers refuse to engage with training and good practice, saying it's unnecessary because they've had a DBS check. NO! A DBS check doesn't automatically make you a good practitioner, so please take that child off your knee!
Also, after Soham, the DBS check does take account of investigations and queries, not just convictions.