Complaints about children accessing smartphone apps increased 300% in a year, with the potential for parents to be hit with bills running to thousands of pounds, a regulator has warned.
Figures suggest two in three 11 to 16-year-olds (63%) have downloaded a free app on to their phones, according to PhonepayPlus, the UK regulator of premium rate telephone services.
But parents who do not talk to their children about what they spend on smartphones can find themselves responsible for bills of "hundreds or even thousands of pounds", according to its report, Children as Connected Consumers.
The report highlights two areas of "particular risk" - free apps and social media linked to smartphones.
It found the risks of so-called free services, particularly malware that charges the phone without the user's consent and in-app billing for "extras", were especially risky for children.
In one case, children as young as 11 downloaded free fake versions of popular games from the Android store such as Angry Birds, Assassin's Creed and Cut the Rope which charged £15 to the user's phone bill every time the app was opened.
The report also highlighted cases where individuals and promoters had taken advantage of children on social media platforms, including one when children aged between 12 and 14 were tricked into "sharing" and "liking" a promotion for supermarket vouchers on Facebook which subsequently misled users into taking part in a premium rate competition.
PhonepayPlus said it took "robust action" in all of the cases and was working with Facebook to ensure that rogue promotions were cut off.
It said parents could take a number of protective steps, including registering the phone as belonging to a child with the mobile network, talking to the network about controls such as pay-as-you-go or blocking certain services, and teaching children to be cautious before posting their number online.
It also urged parents to know which apps their children were downloading and how much they cost, including the price of upgrades.
PhonepayPlus chief executive Paul Whiteing said: "Connected devices will define the age in which today's children live and we are determined to ensure that they can receive the benefits while being protected from the risks.
"Smartphones in children's pockets can burn holes in parents' wallets, so we are working with partners across industry and other agencies to prevent this. This is a real challenge for parents and for us as a regulator but this plan meets that challenge head on."