Panda no Yuubinya-san(1974)

Mio is the daughter of a well-to-do couple who's often left by herself. One day, her precious Panda-chan falls into the river. Upstream is Kenichi, a sickly boy at a hospital who also rarely gets to see his parents. Ken decides to return Panda-chan with a letter, and an exchange begins between the two young children, with Panda-chan delivering messages up and down the river.