How do you read a child's blood presure? My machine won't get a reading off his arm, but just says error.

Have your Dr. check. Blood pressure readings in children can be difficult to obtain. It is important to use an appropriately-sized cuff as the reading will be inaccurate if an adult cuff is used on a small child. Your pediatrician should have multiple different cuff sizes and will use the appropriate size cuff for the individual child so as to obtain the most accurate reading.