Riverside Park: Hippo Playground at 91st Street.

References:

1. Riverside Park: Hippo Playground, NYC Parks Department website: “The property that is now Hippo Playground, on 91st Street, was acquired in 1937 as part of the Riverside Park expansion. The site was developed later that year and was known as the 91st Street Playground.“
2. New Deal Assistance in NYC Parks Department Projects, 1934-43.