In this prospective cohort pilot study, eligible critically ill children aged 3 to 17 years will receive in addition to their usual care, a passive and/or an active mobility intervention, depending on their level of consciousness or cognitive ability. The active intervention will consist of an interactive video game (X Box 360 Kinect), while a cycle ergometer will be used as the passive intervention. The mobility interventions will be applied for a minimum of 10 minutes on day 1, and 20 minutes on day 2. For unconscious patients, or eligible patients who cognitively impaired, the passive intervention will be applied. As soon as the patient is conscious and cooperative, the active intervention will be applied.