The robot arm must be strong enough to lift not only the piece, but its own weight as well. And a 9-year old's finger bone is not an adult femur. Also, from the video, the child's reflex action to having his finger painfully pinched likely contributed some force as well.No chess piece is heavy enough that it requires a level of force that could break a bone
It didn't react to the child. It was its failure to react to a foreign object in the board space, in fact, that caused the event.More important is that why did it even react? Why would it even think or care to act like a human?