I am the CEO of a UK children's charity, and have been on the board of a total of ten companies. I have also been a CEO of seven or eight companies with large retail databases. I have never been asked for a CRB. Some of the charity's employees get CRBs (ones who are put in charge of children) but these things are not generally used for this kind of role. Our trustees are not CRBed.
That surprises me "trustee of a children's charity" is explicitly included in the list of people that should be (or, at least, can be) CRBed. I would suggest that the board seek the advice of the charities commission before making a final decision on CRBs.