It's a Round, Round World! by Ellie Peterson is a great STEM resource for elementary aged children. It is an enjoyable chapter from the history of science that explains how we know that the earth is round. Young scientist, Joulia Copernicus, takes an adventure through time and space to explain the several ways we know about the earth's shape.
Joulia explains that during a lunar eclipse, the earth's shadow is round. Pictures of the earth from space show that it is round. Constellations seen from opposite sides of the earth are different; if the earth was flat, everybody would see the same constellations. At higher elevations, you can see further over the curve of the earth, as demonstrated by the view from a crow's nest on a ship.
The author is a middle school science teacher.