Afterwards, each reported what they ate and researchers calculated their energy intake while sitting in front of each screen. While viewing TV, where they watched either Man v Wild or MythBusters, the average intake totalled 821 kilocalories - or about half the recommended daily intake.
Importantly, the experiment deliberately excluded food advertisements on TV, proving that children could still eat unhealthily in the absence of marketing. When playing Xbox 360 games or going online, the children's intake was less, but still unhealthy, equating to an average 696 and 685 kilocalories respectively.
"We've shown that kids can still eat a lot while playing video games, even though a lot of parents thought they wouldn't, perhaps because they are constantly using their hands.''
In just six years, the obesity rate among children aged between 5-11 has jumped from 8 to 11 per cent.
The research, published in the journal Appetite, will eventually be followed by more work looking at how children eat when using more than one screen at once.