New research from child advocacy group Common Sense Media has found that children's use of tablets, smartphones and other portable gadgets with screens has exploded over the last two years, just as doctors warn too much screen time could be bad for kids.

The group's biannual survey of American parents found an 89% increase in the number of zero to eight year olds who have used mobile devices - growing from 38% in 2011 to 72% in 2013. Even among kids under the age of two, some 38% have used a mobile device for media, compared to 10% two years ago.

Moreover, the amount of time all kids spend with the devices each day has tripled, from 5 minutes to 15 minutes.

At the same time, the American Academy of Pediatrics on Monday reiterated cautions about children's exposure to screens, including mobile devices and TVs. The group said that parents should limit 'total entertainmentscreen time to less than one to two hours per day, and 'discourage screen media exposure' generally to kids under the age of two.

(The Journal reported on the AAP's Monday proposals for families, including a no-device rule during meals and after bedtime.)

The introduction of tablet computers has particularly changed the equation for many families, and the survey found five-fold increase in the number who have access to them. Jim Steyer, Common Sense Media's founder, said the devices are increasingly replacing TVs, storybooks, and even babysitters.

'One-year-olds are now walking up to TV screens and swiping them because they think they are tablets,' he said. 'They are becoming a part of childhood in ways that were almost inconceivable before.'

What do kids do on mobile devices? Mostly play games (63% of them), use apps (50%) and watch videos (47%), the survey found.

Steyer says his organization recommends that parents set limits, though he appreciates that no screen time before age two isn't practical for all families. 'No question about it, the best brain development happens when you speak with kids and play blocks with them and cuddle - all the experiences where there is no intermediation by technology' he said.

And when there are devices involved, he said parents should also try to turn screen time into learning time. 'You have to make wise choices about the content,' he said.