Some interesting points raised, and my take on them:
Why repeat them/ force new checks? People can (and do ) commit crimes AFTER being checked and cleared. Teachers now looking at 3-Years, some LEAs 6 monthly for non fulltimers.
Irregular visits? No need for any checks if the visitor is not left alone with kids- not solo so to speak. The buck stops with the teacher/instructor/unit.
CRB check not 'good enough' for an organisation? Standard Disclosure isn't good enough for working with kids- must be enhanced. That might be the confusion. (Not in the childminder example, granted)