A Nottinghamshire man, whose son spent £3,500 playing on an iPad, has called for tighter rules on online games.
George Holmer, from Beeston, said his son Edwin, 11, "bought gold" in the Arcane Empires game after working out his parents' iTunes account password.
The account was linked to a credit card. iTunes has since refunded the money.
The Office of Fair Trading (OFT) has confirmed it is investigating web and phone games aimed at children.
Mr Holmer said: "It took about three days for the transactions to appear on my online bank statement and there were literally hundreds of them.
"I don't think he realised it was costing money and even if he did that wasn't his focus - he was playing a game and having lots of fun."
Mr Holmer said parents should be able to set limits on how much can be spent on online gaming devices.
"If something is free of charge and specifically aimed at children, who do not have a concept of what money is and where money comes from, you need to make sure that they cannot rack up these kinds of bills," he said.
"I got the money back but I know others who haven't been so lucky."
"Kabam has no control over how children use their parents' devices or the interaction parents have with their children. However, Kabam is pleased to work with its customers in the rare occasions when inadvertent billing errors occur."
The OFT wants to find out if games put undue pressure on children to pay for additional content as part of its investigation.