In the absence of detailed research studies, there are no widely accepted protocols governing the three study treatments. After discussion with physiotherapy and respiratory technicians, the investigators have designed the three study protocols so they are representative of accepted common practice. Each treatment will be administered by a qualified physiotherapist who is familiar with the clinical use of the machine. The treatment will also be monitored by a physician in each of the three treatment rooms. Each child will receive the following study treatments in random order over three consecutive weekdays: