From games inspired by popular TV shows to digital play labeled as educational, children's apps continue to explode on smartphones and tablets, according to the study

But parents may not realize that while their little ones are learning letters and numbers or enjoying virtual adventures, they are also likely to be targeted by advertisers. Ninety-five percent of commonly downloaded apps marketed to or played by children ages 5 and under at least one type of advertising, according to a new study led by University of Michigan CS Mott Children's Hospital.

Published in the  Journal of Developmental & Behavioral Pediatrics , the study reviewed 135 different apps-and it marks the first effort to examine the prevalence of advertising in children's apps.

Researchers found play was often interrupted by pop-up video ads, persuasion by commercial characters to make in-app purchases to enhance the game experience and overt banner that could be distracting, misleading and not always age-appropriate.

"With young children now using mobile devices on an average of one hour a day, it's important to understand how this type of commercial exposure may impact children's health and well-being," says senior author Jenny Radesky, MD, a developmental-behavioral expert and pediatrician at Mott.

Radesky notes that her team found high rates of mobile advertising through manipulative and disruptive methods-with exposure to ads even surpassing time spent playing the game in some cases.

"Our findings show that the early childhood app market is a wild west , with a lot of apps appearing more focused on making money than the child's play experience," she says. "This has important implications for advertising regulation, the ethics of child app design, as well as how parents discern which children's apps are worth downloading."

Ads target youngest players

Researchers detailed experiences with advertisements during gameplay of the most popular children's apps. Although 100 percent of surveyed free apps included advertising content (compared to 88 percent of purchased apps), the ads occurred at similar rates in both types of apps categorized as educational.

Ad videos interrupting play were prevalent in more than a third of all analyzed apps and in more than half of free apps. In-app purchases were also present in a third of all apps, and in 41 percent of all free apps.

This discrepancy worries Radesky: "I'm concerned about digital disparities , as children from lower-income families are more likely to play free apps, which are packed with more distracting and persuasive ads."

Some ads were particularly deceptive: Familiar commercial characters would appear on-screen to remind players that paying for certain in-app upgrades and purchases would give them access to more appealing options and make the game more fun.

Overt banner ads covering the sides or top and / or bottom of the screen during gameplay were also present in 17 percent of all apps and 27 percent of free apps. Some banners promoted adult-appropriate apps that required a user to watch the full promo before a box could be closed.

Authors note that prior research has found children ages 8 and younger can not distinguish between media content and advertising-and that fewer regulations apply to advertise in than on television-which raises further ethical questions around the practice.

"Commercial influences may negatively impact children's play and creativity," Radesky says. "Digital-based advertising is more personalized, on-demand and embedded within interactive mobile devices, and children may think it's just part of the game."

The findings, Meyer says, suggest the need for heightened scrutiny of apps in the educational category . "We hope further research will help us better understand the consequences of digital media," she says.

Child consumer advocacy groups, led by Campaign for a Commercial-Free Childhood , plan to file a complaint with the Federal Trade Commission about the study's findings.