The study and measurement of grasping actions and forces in humans is important in a variety of contexts. In infants, it can give insights on the typical and atypical motor development, while it poses functional and operative requirements that are not fully matched by current sensing technology. Novel approaches for measuring infants' grasping actions are based on sensorized platform usable in natural settings. A new set of instrumented toys has been designed for the assessment/stimulation of upper limbs of infants between 4 and 9 months. A purposive biomechatronic gym has been developed by integrating pressure and force sensors and visual/auditory stimulations to the usual gym structure and hanging toys (cow, flower and ring puppets), so that the infants' actions on the gym can be monitored, measured and stimulated. With the developed system, a longitudinal clinical validation has been carried out with seven healthy infants. From data analysis it is possible to identify a trend in manual forces development and this result confirms the usefulness of the system proposed as a clinical tool for monitoring infants' grasping development.