The US government has sued Amazon.com  for allowing children to collectively incur millions of dollars in purchases on the credit cards of their unsuspecting parents while playing mobile applications such as Tap Zoo and Ice Age Village.
The action, filed by the Federal Trade Commission (FTC), seeks to make the online retailer refund money spent without parental permission and to end Amazon's practice of allowing purchases without requiring a password or other mechanism that gives parents control over their accounts.
The unauthorised charges were often associated with children's apps, such as games, that could be free to download but allowed in-app purchases by buying "coins" or other digital products with the credit card associated with the device, the FTC said in its complaint.
The complaint named Tap Zoo and Ice Age Village, in which children manage a zoo or an ancient town. To do that, they can purchase digital items that often cost real money.
One user put a review on Amazon.com  in July last year complaining that Tap Zoo was a "cash trap" and said his son had spent US$65 on it without permission. The game now tells parents how to disable the purchasing function.
The apps run on Amazon's Kindle Fire, Kindle Fire HD and devices that use Google's Android operating system.
The FTC settled a similar case with Apple in January. Apple agreed to refund to customers at least US$32.5 million in unauthorised charges made by children and to change its billing practices to require consent from parents for in-app spending.
Amazon lawyer Andrew DeVore earlier said that an FTC threat "leaves us no choice but to defend our approach in court".
The FTC said in its lawsuit that Amazon had responded to complaints about unauthorised charges by requiring passwords for large purchases in 2012.
That was extended to all purchases last year, but once a password had been entered, a purchase window remained open for up to an hour, meaning that further charges could be made without parents' knowledge, the complaint said.
Some parents said their children spent hundreds of dollars without their knowledge, the complaint said. Amazon billed for the in-app purchases and kept 30 per cent of the charges, the complaint added.