Experts say climate change and harmful advertising encouraging fast-food consumption and under-age drinking are putting children at risk.
The UK was ranked among the top 10 countries in the world for the overall health and wellbeing of children.
However, it fell behind in safeguarding the environment for their future.
The report by the WHO, the United Nations Children's Fund (Unicef) and the Lancet Commission, ranked 180 countries on the likelihood of a child being able to "flourish", focusing on health and wellbeing factors such as education, nutrition and child mortality.
Countries were then also ranked on their carbon emission levels.
Some 40 child-health experts warned progress over the past two decades was "set to reverse" if radical changes were not made by governments around the globe.
"Every child worldwide now faces existential threats from climate change and commercial pressures," said former Prime Minister of New Zealand, Helen Clark, who co-chairs the commission.
"Countries need to overhaul their approach to child and adolescent health to protect the world they will inherit in the future.
The experts warned a 4C rise in global temperatures by 2100, in line with current projections, would result in "devastating health consequences" for future generations - a rise in ocean levels, heatwaves, severe malnutrition and a spike in infectious diseases such as malaria.
"More than two billion people live in countries where development is hampered by humanitarian crises, conflicts, and natural disasters, problems increasingly linked with climate change," said minister Awa Coll-Seck, from Senegal, who co-chairs the commission.
While the world's poorest countries were found to be among those with the lowest greenhouse-gas emissions, they were deemed most likely to be exposed to the negative impacts of climate change.