Riverside Park: Dinosaur Playground overlook and service building, comfort stations under the arches.

References:

1. NYC Parks Dept press release of October 11, 1937: “North of 96th Street, the completion of the major play area of the development is dependent upon the progress of bulkheading and placing of additional fill" (which confirms this playground was part of the 1934-37 West Side Improvement).
2. New Deal Assistance in NYC Parks Department Projects, 1934-43.