The Brambles kept the screat or fact away from his child because Harold was very gentle and sophisticated in behaviour. Mrs. Bramble had fear that it will be difficult for Harold to accept the qualities or behaviour of his father. So , the Brambles found it better to hide this secret of being a " Professional Boxer".
In case of them , Yes I agree they were satisfied as they both were having fear of acceptance of the qualities of his father.
If I am in the place of Harold's Father , I should tell the truth to Harold that I m a " Professional Boxer" , so that Harold would not think that I had hidden anything from him.