Advertising messages on networking sites, games — known as "advergames" — and other social media create an impact in children and their dietary habits.
Children in Europe are bombarded with hidden digital advertising and marketing promoting fatty, sugary and salty foods that is damaging their health and adding to the region's obesity problem, World Health Organisation experts said.
"Our governments have given the prevention of childhood obesity the highest political priority, (yet) we consistently find that children - our most vulnerable group - are exposed to countless numbers of hidden digital marketing techniques promoting foods high in fat, sugar and salt," said Zsuzsanna Jakab, the WHO's regional director for Europe.
"Often, parents do not see the same advertisements, nor do they observe the online activities of their children; many therefore underestimate the scale of the problem," said the WHO.
About two-thirds of children who are overweight before puberty will be overweight in early adulthood, and an estimated 25 percent of school-aged children in Europe are already overweight or obese, said the report.
Overweight and obese children are likely to stay obese into adulthood and more likely to develop chronic illnesses such as diabetes and heart disease and cancer at a younger age.
Gauden Galea, a WHO Europe expert on chronic disease and health promotion, said allowing advertisers and the food industry to target children like this could have "huge health and economic consequences".
The WHO Europe report said that since there is little effective regulation or control over of digital marketing, children are often exposed to powerful and targeted online marketing via digital platforms that collect personal data. Digital marketing can engage children in emotional, entertaining experiences and encourage them to share these experiences with their friends, it said, describing this as "a dubious cocktail when used to promote unhealthy foods".