Gigi C. answered 08/05/20
Passionate and Disciplined German Speaker with Roots in Bavaria
Guten Morgen, Trey!
"Der Fels" and "der Stein" function pretty similarly to how "rock" and "stone" function in English. Der Fels has more of a geological or natural context and often refers to huge rock faces that you'd see on a mountain. Der Stein, one the other hand, usually describes a smaller piece of rock, often one that was shaped by human hands.
A really simple explanation that I would use with small children is that "Ein Stein ist ein Stück Fels/A stone is a piece of rock."
I've also provided you a video, because for some reason I couldn't insert images into this comment! The video has verbal explanations, definitions provided from a german dictionary, and two images to compare.
I hope this helps. Viel Glück!