This is "The Children of Primrose Lane" by Noel Streatfield.
London children of three different families, in a German spy chase --
necessarily clandestine since they do not want to implicate a
soldier-father in revealing troop movements. Having decided that the
stranger, for all his believable story, is a spy, they plot to keep him
under surveillance, destroy any chance he may have to communicate with
confederates, and to give him no opportunity to pick up any other
information. They also have to evade two other strangers, who may or may
not be spies or British Secret Service...and through their combined
talents, intelligences, plans, they hang to their man, even in a chase
to the East Sussex downs, and capture him for the government."