Operation Kingfisher was an escape line organised by the French Resistance in WW2 to smuggle downed Allied airmen to Spain via the French canal network. In this book the teenage brother and sister of an Anglo-French family who have been stranded in France by the war use it in the opposite direction (i.e. S-N) to try to reach Switzerland and so join their father in England.
The attempt is soon blocked, and the pair spend most of the book with the maquis in the highlands of the Morvan, ambushing Germans and collecting intelligence. Eventually they meet up with their father who, unknown to them, is an SOE agent, and he gets them back to England.
I am not sure whether this is meant to be a young adult book. Not only are the hero and heroine both teenagers, but they are so wholesome, the violence is sanitised, there is little moral ambiguity, and the maquis usually win with minor casualties. There is no hint of the tragedy to come when the maquis over-reach themselves and try to set up a liberated area in the Morvan.