The vast majority of young children in the United States are using mobile devices and for much longer periods of time, with an even greater number of babies being exposed to the smartphones and tablets that have become a bigger part of family life, according to a survey.

Seven out of 10 children younger than 8 have used a mobile device, a figure that has doubled in two years, according to a report to be released Monday by Common Sense Media.

Those children are spending triple the time on mobile devices — about 15 minutes daily — that they did in 2011, with playing games, using educational apps and watching videos among their most popular activities, said the San Francisco-based child advocacy group.

Four out of 10 children younger than 2 are also using mobile devices, a jump from one in 10 two years ago.

The findings come amid increased concern over the time children spend online as families snap up gadgets, game consoles and computers. The American Academy of Pediatrics (AAP) has advised parents to limit their children’s time in front of screens, which it says may lead to attention problems, exposure to inappropriate content and obesity.

A big business has emerged out of selling apps and devices for children as young as newborns, promising educational and entertainment benefits that some public Internet advocates say are misleading.

The federal government created privacy rules for children 12 and younger, but some critics say regulators and tech companies need to do more to protect young users from predators, mature content and a glut of advertising. The AAP suggests children younger than 2 avoid any media use, and authors of its media policy say there is not enough research to support companies’ claims that smartphones and tablets can educate babies.

“This shows for the first time the development of a true digerati generation from cradle onward,” said James Steyer, chief executive of Common Sense Media . “Used wisely, technology blended with good content can be a positive form of media, probably better than passive TV viewing. But there are still dangers of too much screen time, and this should be a wake-up call to the tech industry and to parents.”

The national survey of 1,463 parents found that television viewing continues to take up most of the time that children spend with media.

Six out of 10 children watch TV every day, compared with 17 percent of children who use a mobile device daily. The number of children using mobile devices daily doubled from 8 percent two years ago. And 28 percent of children use a mobile device each week.

The growth in mobile-device purchases in U.S. homes has put such technology increasingly in the hands of children. Three-quarters of every U.S. home has a mobile device, compared with about half of homes two years ago.

That has made smartphone and tablet use a greater part of the family experience. By the time children enter elementary school, the vast majority have used mobile devices. Eight out of 10 5-to-8-year-olds use mobile devices, compared with five out of 10 children two years ago.

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