The UK's children now spend more time online than they do watching television, official figures show.
Youngsters aged between five and 15 now spend around 15 hours online each week.
This figure is up by an hour and 18 minutes on last year and overtakes the amount of time they spend in front of the TV for the first time, Ofcom said.
Even three and four-year-olds are spending an average of eight hours and 18 minutes a week online, up by an hour and a half on last year.
YouTube has become one of the most popular online destinations for children to watch content, with 73% of five to 15-year-olds and 37% of pre-schoolers regularly using the video site, typically for TV-style cartoons and mini-movies.
To make time for this, children have cut the time they spend watching TV from an average 14 hours and 48 minutes last year to 13 hours and 36 minutes.
However, nine in 10 children still watch TV, generally every day and between 6pm and 9pm.
Ofcom's study found that 34% of pre-schoolers own their own media device such as a tablet or games console and 55% use a tablet and 16% owning one of their own, up from just 3% in 2013.
But they show a preference for smartphones over tablets as they get older, with one in three eight to 11-year-olds and eight in 10 of 12 to 15-year-olds now having their own smartphone.
However, a separate piece of Ofcom research, also published today, shows that reading is the third most popular activity among primary school children, ahead of watching online video clips, instant messaging and watching music videos.
And many families still enjoy the traditional bedtime routine, with one in five children aged six to 11 reading at 7.45pm - the highest peak for reading during the day.
Most families believe that their child has a "good balance" between screen time and other activities, with 64% of children aged 12 to 15 and 65% of parents of this age group believing that the balance is "about right".
Ofcom director of market intelligence, Jane Rumble, said: "Children's lives are increasingly digital, with tablets and smartphones commanding more attention than ever.
"Even so, families are finding time for more traditional activities, such as watching TV together or reading a bedtime story."