A branch of psychology supposedly founded by Wilhelm T. Preyer (1841-1897) that typically studies the behavior of children at key ages or makes comparisons between adjacent ages using experimental methods in order to tease out age-specific functions, processes and mechanisms that may be deemed to be cognitive, emotional, social etc. in nature. Often it is used interchangeably with developmental psychology, but strictly speaking there are differences between them, the main one being that the chief concern of developmental psychology should be the study of both intra- and inter-individual differences as a function of age. Child psychology as generally practised does not deal with change in this way, nor with the identification of developmental transitions between states or stages, but at most with age differences in a variable-centered approach.
See Child development, Developmental psychology, Intra-individual differences, Mechanism, Process, Transition, Variable-centered approaches