The Secret Seven by Enid Blyton

A group of resourceful children form a secret club with badges and a password, and during a snowy holiday they notice suspicious goings-on near an old house. With keen observation, some stealthy eavesdropping, and help from their loyal dog, they follow clues that reveal a missing child being hidden by criminals. Working together, they alert the authorities and stage a daring rescue, proving that courage, friendship, and teamwork can crack real mysteries.