"Sprock-it" is a hand-sized robotic character that encourages full-body interaction and engaging mental play. Through social and physical interactions with the system and with each other, children influence the character's mobile and responsive autonomous driving behavior. Enabling constructionist activities and participatory adaptive design the system employs RFID tags and readers, wireless Bluetooth modules, Atmega microprocessors within Arduino's development environment, along with a wearable interface and a variety of physiological/activity sensors. Sprock-it has been developed in collaboration with LEGO and the MIT Media Lab as a concept that represents a new strategy for LEGO by updating the original LEGO mantra of "the little constructor" to "the little interactor". This paper presents the system development of Sprock-it.