STEM stands for science, technology, engineering and math. STEAM is the same four subjects with the arts added, so drawing, music, movement, design and storytelling become part of how kids learn science and math rather than a separate class at the end of the week. For young children, most of the time STEAM is the better fit, because the arts give little kids more ways into the same big ideas.
That doesn't mean STEM is wrong or that STEAM is a magic upgrade. It means a 4 year old learns about patterns, shapes and cause and effect through her whole body and imagination, and STEAM makes room for that.
