Not familiar with Margaret Campbell Barnes, but I'll give her a look-see. Another good one is Frances Hodgson Burnett (she goes way beyond "The Secret Garden" and "Little Lord Fauntleroy" - she wrote adult novels too. "A Lady of Quality" is very good.)
Jan Westcott was another that I read. I had a lot of books in paperback but they have died a horrible death so many years later.