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).