everyone suggest farmville, and i can't agree more. but just make sure your children don't play too long. give him/her a time limit and try don't leave him/her play alone (well, if they feel bored with farmville, they can browse FB to find another game that more challenging but won't suit to them. so you need to stay close to your children )
children should not play games on internet as they bacame addict of this and it suffer thier results.
In my views, dnt encourage ur kids for online games. Motivate them for outdor games. It keep them fit.