Some years ago, it was revealed that he not only had a fee negotiated by his representative but he took it. When this became public, his ungracious reply was that the cameraman got paid, so why should he not be.
Given the bad publicity from that, it is unlikely that celebrities on Children in Need take a money for appearing, whatever their agents, looking for a percentage, say. The benefit for them is the PR one, of looking good to the public (which is not to say that they wouldn't volunteer to help a charity)