Walking any sort of distance should not cause any problems in later life, your joints are designed to last the life time of your body. It only becomes a problem if they are carrying extra weight either in bags or because of being overweight as this will cause over due stress on the joints.
The more they can walk the better! There are too many children today who just spend their entire lives sitting in front of screens at school and at home. They get no exercise and no fresh air.
New Labour said a few years ago that this generation of children at school are the first who are not expected to live as long as their parents. The two reasons Blair cited were poor diet and lack of exercise.