The new Government has halted the progress of the 'vetting and barring' scheme. The rules in the next three paragraphs state what was meant to happen before this was announced. (It's simply a case of 'watch this space' to see what will actually happen in future).
As of the 26th of this month, volunteers for such activities as yours could have applied for ISA registration.
As from November, new volunteers would have had to apply for ISA registration.
As from next year, existing volunteers (including those who've undergone CRB checks, such as yourself) would have been required to register with the ISA.
http://www.isa-gov.org.uk/default.aspx
Chris