Just occurred to me, since a so called friend did tell dd aged only 5 that there was no Santa, 'it's just a fairytale'.
Thank heaven I thought to tell her at once that so called friend had probably been so naughty when she was little that Santa never came, so it was no wonder she didn't no believe in him. Should add that I was helped by the fact that friend had form for rowing, effing and blinding, inc. in front of dd at least once.
This worked like a dream, so thought I'd share in case anyone else's dc encounters a misery wanting to spoil the magic.