OK, eu fiz um googling intenso, e é quase certo que nós dois estávamos pensando em Knight's Castle por Edward Eager (Texto disponível em Gutenberg no Canadá) . Da sinopse em GoodReads :
When the ancient toy soldier comes alive, the Old One grants Roger's wish to adventure in "yeomanly" Sherwood Forest. Will Roger earn his second wish, to save his father? Need for an operation brings Roger 11 and younger Ann to stay with bossy cousin Eliza and Jack. Can the children convince Ivanhoe to marry Rebecca over Rowena, and conquer the castle - in pjs?