I just got off the phone with the charities commission (should
probably have done that sooner and saved all this discussion!), and
the woman I spoke to didn't think CRB checks would be required just
for handling the details of children, they'd only be required if the
trustees are actually going to be in contact with the children (AGMs
and things don't count, since those are all in groups). Anyone that is
going to be going into schools, or similar, would need to be checked,
of course. She did recommend that the charity have a specific policy
saying that trustees wouldn't have any contract with children (without
being checked).