Reptile Gardens is a long-running family attraction on U.S. 16 south of Rapid City, between town and the Mount Rushmore route. Snakes, crocodilians, giant tortoises, birds, and botanical displays make it one of the area's most reliable kid-friendly stops.
It is commercial, yes, but well established and easy to fit into a Black Hills day. If you are travelling with children, it may buy more goodwill than another scenic overlook.