Online games that are just for the kids often have a two tier system. Kids can earn e-money (shells, bits, whatever it is to make it look it isn't e-money) through participation, but ultimately to make real progress they (their parents) need to buy into the games. Games are often played by spending their e-money and if they win the game they earn more e-money.
None of the answers above are relevant to me, however, I wouldn't like my grandchildren to use this site or any that encourage any form of gambling and/or spending money online. In fact, I wouldn't let them go on anything unless it was for school homework research. Too many bug-eyed kids these days (oh, make that adults too). Get outside kids!
As I am sitting at a computor reading this, can't totally disagree. But computor use by the young can lead to a good job in the future. It's all about balance. Everyone needs activity and exposure to vitamin D so take up some exercise and fresh air. 30/60 mins once or twice a week is fine.
They shouldnt feel they have to pay to progress. My kids play games but know they cannot make any online purchases. Therefore if thats expected of them they are most likely going to ditch that game and move on.