I was recently at a lecture by Dr Dashti, consultant adult and paediatric spinal surgeon. He was talking about the increased incidence of back pain amongst children and teenagers directly due to the playing of video games or sitting in front of a computer for more than 2 hours a day. He said on top of this school children can carry 20% of their bodyweight around in their schoolbags as the majority of high schools now don't have lockers for students to put their books in.
He obviously sees young people in his clinic with underlying issues that need treating but was stressing how inactivity amongst children and teenagers can lead to poor posture, back and neck pain.
I am always asked for advice from people re their children experiencing back pain and always advise they get it checked out by their Gp first and foremost. If no underlying issues are suspected then they need to have a look at activity levels and lifestlye choices. Find a physical activity for the child/teenager that they enjoy anough to do regularly and encourage them to stick with it. Do it with them if it's appropriate eg:swimming, tennis, badminton,football or even just a regular walk to school.
I run a Pilates class specifically for teenagers from 13 yrs old and we work with swiss balls and other equipment so it is always challenging and fun.I started this class when my teenagers were doing sport and experiencing aches and pains along the way. It soon grew and is verp popular with local students sporty or otherwise.
Younger children need to be coaxed away from their computers and video games (unless it's an activity based game of course!) and stay active. Regular PE sessions at primary school should give children all the tools they need to improve thier posture ie: balance, co-ordination and even core strength.