Repetitive strain injury (RSI)
Find an ergonomic chair that will keep your child's body at the right height. Ideally if the position and height of all tools is the right one (including the chair, desk, keyboard, mouse, monitor) everything will come almost naturally and will require little attention from your children to keep the position right. Check the feature Computer Posture Reminder of Children's Care software.
Chair & desk - The chair should support upper and lower back of your children and allow their feets to be firmly placed. The height of the desk is not so important as long as the chair compensate it. A chair with adjustable height is the perfect choice.
Monitor - The upper corner of the monitor should be at the same height as the eyes
Keyboard & Mouse - The keyboard and the mouse should be at a height and distance from the body, avoiding any tension in wrists or elbow. Make sure your children are not holding the mouse tightly.
Step back and look at your child while using the computer. If you feel that everything in his/her body is relaxed and natural than probably everything is fine. If you feel tension or that something is not right, find what is the reason and correct it.
Encourage your children to take regular breaks. A short exercise during the break can have an enormous positive effect. Check the feature Regular Breaks of Children's Care software.