Blizzard has made a change to parental account controls that, by default, set kids' accounts to 'not allowed to buy stuff'.
This applies to the in-game World of Warcraft shop and to real-money purchases in WOW, Hearthstone and the Auction House in Diablo 3 (which is being closed on 18th March, good riddance).
"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 - yes I set my daughter up with a Hearthstone account so I could at least win against someone.
"No action is needed to keep this default setting and prevent in-game purchases."
You can of course enable in-game purchases again by logging-in to Battle.net, clicking Retrieve Parental Controls, following the various on-screen instructions and selecting 'allow in-game purchases'.