Questions: Did Piaget study the question of when children learn that they do not directly feel the  pleasure or pain of other creatures? This is related to the idea of the possibility of solipsism as resulting from this realisation. Piaget is usually said to be  testing the Kantian theory of ideas such as time, space, causality being innate. Did he consider to test the windowless monad theory of Liebniz (Monadology)?

Answer1: See Piaget's work on Possibility and Necessity.

Answer2:His research finding refuted Kant's a priorisms and Liebniz' monads if that's what you mean. See Principles of Genetic Epistemology and Structuralism.