Just in time for Christmas, a number of new games are hitting the app marketplace, but tech experts worry parents are neglecting the fine print and will get an unwanted present when they get their next phone bill.

Though new games like Angry Birds Go! — the latest instalment in the popular Angry Birds series — are free, players sporadically receive the option of installing add-ons, many of which come with a price tag.

Angry Birds Go!, the latest in the wildly-popular Angry Birds series of games, is free in Apple's App Store but has the option for gamers to make real-life purchases inside the game. (Rovio Entertainment Ltd.)

The fees, called in-app purchases, can quickly add up.

David Fewer, an Ottawa-based lawyer who specializes in technology, says the fees come as mobile gaming app companies are changing their business models.

"The game is free, but they make money off of people levelling-up or buying the magic sword they need to complete the level," he told CBC News.

"The enormous problem for most parents is that they don't realize or don't understand that the game is built on this model, and often the children themselves don't understand that they're spending real money in purchasing this."

He says app developers need to do a better job of educating parents about in-app purchases, and he advises parents to get in the game.

"You have to exercise some supervision over what they're purchasing and understand the business models the various games are implementing," he said.

Fewer also said there is some leeway, and companies like Apple or Google have reimbursed some accidental purchases.