The simple answer is 'No'.
Such an activity doesn't fall within existing legislation relating to regular activities, involving minors, of over 2 hours duration, such as play schools and organised holiday activities. (Those type of activities require registration with the local authority, which in turn involves a requirement for staff to be CRB checked).
At present, less formal activities with young people don't result in any statutory obligation to have adults CRB checked. Of course, organisations such as the Scouts can, and do, make their own rules but there's no legal requirement for them to do so. Further, if someone's CRB check reveals criminal convictions, it's entirely up to the organisation to decide whether they will permit that person to work with children. (Under the present rules, the CRB can't directly ban anyone from working with children).
The new 'vetting and barring' scheme will give the CRB the direct power to bar someone from working with children. However (although certain measures apply from 12/10/09) it doesn't come fully into effect until late next year. Even then, it will only apply to regular contact with young people. 'One off' activities, such as the one you refer to, will not be covered.
Chris