This paper describes an interdisciplinary approach to the assessment on infants' behavior, with a focus on the technology. The goal is an objective, quantitative analysis of concurrent maturation of sensory, motor and cognitive abilities in young children, in relation to the achievement of developmental milestones. An instrumented block-box toy specifically developed to assess the ability to insert objects into holes is presented. The functional specifications are derived from experimental protocols devised by neuroscientists to assess spatial cognition skills. Technological choices are emphasized with respect to ecological requirements. An ad-hoc calibration procedure is also presented which is suitable to unstructured environments. Finally, preliminary tests carried out at a local day-care with 12-24 months old infants are presented which prove the in-field usability of the proposed technology.