The Children of Primrose Lane by Noel Streatfeild.
 
During the suspenseful years of World War II, six children living on Primrose Lane find themselves with more excitement than they bargained for. Their usual hangout, an empty house, is unexpectedly occupied by a mysterious stranger. With wartime suspicion high, the children worry this man might be a dangerous German spy.
Torn between loyalty and fear, the children embark on a secret mission. They must decide whether to expose the stranger or hide him, all while keeping their suspicions a closely guarded secret. This thrilling tale of wartime intrigue and childhood bravery will keep you guessing until the very end.