Caroline Hartley and the Great Adventure — D.J. Robinson

Caroline Hartley has a magic key. It has allowed she and her brother Martin to secretly step through an attic door and time-travel back to 1912, at a castle where they are staying with their father in Central Europe.

One evening, whilst in 1912, the children discover Caroline's magic key has gone missing. A frantic search with their friends fails to find it. Only the servants might know where it is, but some of them have taken time off. Caroline and Martin are stuck in 1912, unaware of the great adventure that now awaits them…