Mere Christianity question

(I'm sorry about labeling this book but it wouldn't let me post without tagging a book reference) I need help in finding a book that my mother remembers but never was able to buy. She doesn't remember the title or the author, however she remembers the plot, cover, the about time frame of when she read it, and where she remembers finding it in the bookstore. The book was found in the science fiction/fantasy section in a bookstore in north bend/coos bay area (B. Daltons or Waldons books). She says between 1979-1982 is when she believes she read (in high school) although she says possibly later when she attended community college but not anytime after 1984. The cover featured a woman riding with a young boy (the boy being in front of her) on horseback and a young girl riding a pony behind her. PLOT: The story was after the fairy-tale happily ever after when the prince and princess married and had children. One day the prince (Probably a king but mom is unsure) goes missing and the princess (possibly queen) goes on an adventure to find him bringing along her children. My other believes there were two children a young girl (the older child) and a young boy (the youngest) however she says there may have been just one child or three but is fairly certain it was two children. How my mom phrases it is: "Mom goes on adventure bringing her kids". Please help in my quest to find this book for my mother.