The best posture for kids when playing games

If you are concerned about long periods of sitting at the computer, get them up and moving periodically.

Better yet, limit the time available for them to be at the computer and get them doing something else like playing outdoors, doing chores and doing homework the other times.

http://ergo.human.cornell.edu/Pub/HFPresentations/NEEkidsweb.pdf

Good posture won't do anything to alleviate the effects of not not doing all the things kids could be doing instead of sitting in front of a computer for hours.