Flußfahrt mit Huhn (1984)

Johanna doesn't think much about her Cousin Robert -- he seems to be rather a rascal. But when their parents go on a vacation, both children have to stay with their grandfather somewhere in Germany. Johanna finds out that Robert has different plans: He wants to discover a passage to the North Sea. And soon Johanna finds herself on a boat with three boys and a chicken called Gonzo, meeting all kinds of adventures on the river.