Studies of child development of tool use precursors show successive but overlapping phases of qualitatively different types of behaviours. We hypothesize that two mechanisms in particular play a role in the structuring of these phases: the intrinsic motivation to explore and the representation used to encode sensorimotor experience. Previous models showed how curiosity-driven learning mechanisms could allow the emergence of developmental trajectories. We build upon those models and present the HACOB (Hierarchical Active Curiosity-driven mOdel Babbling) architecture that actively chooses which sensorimotor model to train in a hierarchy of models representing the environmental structure. We study this architecture using a simulated robotic arm in a 2D environment. We show that overlapping phases of behaviours are autonomously emerging in hierarchical models using active model babbling. To our knowledge, this is the first model of curiosity-driven development of simple tool use and of the self-organization of overlapping phases of behaviours.