Critically ill patients are often confined to bedrest, because of the nature of their illness. However, it has now been shown that prolonged bedrest and immobility, particularly in critically ill adults and children, can lead to serious outcomes such as muscle weakness and prolonged stay in the intensive care unit (ICU). Research in adults has demonstrated that preventing immobility and introducing exercise and rehabilitation in adult patients early during their ICU illness can prevent these serious outcomes, reduce the duration of hospital stay, and enhance their recovery after they leave the ICU. The purpose of our research is to investigate if early mobilization and rehabilitation can also be performed in critically ill children, and eventually evaluate this results in beneficial outcomes in these children, as it has been shown to be in adults. However, given that children are not the same as adults, but have varying abilities to understand and participate in activities when they are sick, the investigators first have to evaluate the most appropriate and safe ways of helping children "exercise" while they are in the ICU. This objective of this study is to evaluate if 2 methods of rehabilitation (a stationary bike and interactive video game) can be implemented in critically ill children. These methods have been used in hospitalized children and in critically ill adults. The investigators will evaluate if these methods are safe for critically ill children.