Blizzard has changed the default settings in the parental account controls for Battle.net, that sets it to "not allowed to buy stuff." This change applies to all games offered by Blizzard that offer some sort of micro-transaction services or auction houses including WOW, Hearthstone, and the Auction House in Diablo 3.
"We're making this change to ensure that Parental Controls users will have more control over real-money purchases in Blizzard games," explained Blizzard in an email to customers with children playing Blizzard games. "No action is needed to keep this default setting and prevent in-game purchases."
Those who want to allow their children to make in-game purchases can change the settings by logging-in to their Battle.net accounts, clicking "Retrieve Parental Controls," and selecting "allow in-game purchases."