Infants with Down syndrome show an altered pattern of motor development at early childhood. Treadmill-walking training can be used to promote the earlier attainment of motor milestones in infants with locomotion deficiencies but quantitative data on their motor development are limited to gait laboratory studies. Our purpose was to develop, validate, and test a low-cost portable system for detecting infant steps on a treadmill while training. The system includes five infrared distance sensors, which were placed on a motorized treadmill to record the location of the feet of the infant during walking and thus measure his/her step length and cadence. The system was validated using synthetic objects, with a healthy 13-month-old infant. Pilot studies were then conducted in additional five infants with Down syndrome (aged 11–28 months) to determine step length (17.5–22.3 cm) and cadence (0.33–2.16 steps/s) at baseline, as well as at follow-ups 1 month and 3 months after the first trial. Measurements were repeatable per session and agreed with values reported in the literature. These pilot studies indicate the potential utility of the present system in quantitative monitoring of the process of acquisition of initial gait in infants with Down syndrome at the care facility where routine therapy is given.