First, how old are the children?  Second, they are the children, you are the parent.  It is not "being demanding" to set guidelines and rules.  Computers have taken the place of the T.V. but ground rules must still be placed.  Educational computer games can be beneficial to children, but still should only be played AFTER homework is complete.  Games that have no educational benefit, is a privilege that should be earned (just as TV time) and therefore, privileges can be removed if grades drop, chores or studies are NOT complete, etc.  You are the parent, it's your rules and your place to set appropriate guidelines for your children to follow.  It's not "being demanding" it's "being responsible" and they too need to learn to take responsibility of what is expected of them.  Best wishes!

Well rules are rules  and if he dosnt listen to you then tell them to go to their room. yhats what my mom says

You can't.  You are the parent and you are right.  That is all that is necessary.  Go ahead.  Be demanding.  They'll thank you for it in 10 years.

Okay. You are a mother. Sometimes, you need to be demanding. Sometimes it's the only way to show your kids that you mean it. If you want to change their computer habits, block bad websites and such content. Only let them go on their fun websites occasionally. Only let them access educational things.

Use the computer games as reward for work. Make them a credit card. Punch holes every time they do homework or chores or whatever you want them to do. Limit the time they spend during the week. Get them outside to play and get fresh air. You go for walks with them, play ball with them, bike rides etc.
We are, as a nation, spending way too much time with computers, fancy phones, WII etc. Invite friends over after school to do art projects or sports.

I suggest that you encourage your kids by introducing them to educational games, and cutting down on the ones which only seem to waste time.

Brain trainer, big brain academy, bbc learning games (see the link) are all great, but are not the only options.

There a many game series such as Civilization, Age of Empires, Masters of Olympus, Sherlock Holmes, and Railroad Tycoon which can help anyone learn. Many game elements follow historical events, which can teach you history and geography whilst having lots of fun.

Search Google for learning games
http://www.funbrain.com/
http://www.miniclip.com/games/en/learning.php

Get them games such as scrabble, crosswords, and Sudoku.

Challenge them to beat your score on the learning games.

Offer time on the X-box or PS3 as a treat/reward for studying hard.