BERLIN—Tilda Geyer got away from her parents on a trip to a lake last summer. She and 10 friends slept in tents, rode horses, picked berries and flew on a rope swing over the water. At night, they sang around a campfire.

Tilda was just 4 years old. The campout was for kindergarten.

German kindergartens aren’t for crybabies. While U.S. preschoolers...