A mother is urging other parents to be wary, after her twin seven-year-olds racked up a $3,000 bill when she let them download and play an app from iTunes that she thought was free.

"Our house rule is never download an app without permission," Paula Marner told CBC News.

"And they didn't download an app."

While on a trip to England, Marner let her two boys play a free app on the family's Apple iPod and iPad devices.

What she didn't know about was something called in-app purchasing.

How to protect yourself

Click through the slides above to see how users of Apple (left) and Android devices can secure themselves from inadvertent in-app purchases

"To make a long story short, it was not fraudulent activity or criminal activity that I thought was happening from the U.K.; it was actually my seven-year-old sons who were playing a game while I was gone called Clash of Clans," Marner said.

Clash of Clans is one of the top-grossing iTunes apps of all time.

Downloading the application is free, but within the game, you can spend cash on in-app purchases.

In other words, the good parts of the game cost money.

Elias and Malachy didn’t know that — and neither did their mother.

She says the boys were putting in the password and being prompted to make purchases that ranged from 99 cents to $99.

"So that kept coming up consistently and they kept tapping it, because it's just tap purchase, tap purchase, tap purchase," she said.

Too busy to complain

Technology law expert David Fewer of Ottawa said app makers may be banking on parents who are too busy to bother with complaints.

"The $3,000, $5,000, $10,000 bills, people are really going to complain about that when they show up, because it's pretty noticeable on your credit card statement," said Fewer, who is the director of the Canadian Internet Policy and Public Interest Clinic.

David Fewer is the director of the Canadian Internet Policy and Public Interest Clinic. (CBC)

"But if it's just $20 or $40, parents are going to go into that cost-benefit calculation. 'Is it worth my time to complain about this, or should I just pay it and tell my kid not to do that again?'"

Despicable Me 2 is the big kids’ movie this summer, and the app is a big hit as well.

At one point, it was a No. 1 download — and a free one. But in-app tokens can cost up to $50.

St. John's-based app developer Levin Mejia says targeting children is wrong.

"There are a lot of in-app or app developers who are taking advantage of in-app purchases, banking on the fact that kids are going to be given an iPad and the parental permissions won't be set out correctly," Mejia said.

‘I think it’s very deceptive’

It's been a month since Paula Marner started her virtual journey, and she’s not happy.

"I think it's very deceptive," she said. "I accused them of a little bit worse, but it's just greed."

Apple is working to settle a $100-million US class-action lawsuit over in-app practices.

Marner is not part of that suit, but she is getting her $3,000 back.

She wants Apple to crack down on game-makers, to protect families from this kind of shock.

Marner also wants in-app purchasing disabled on Apple devices.