Annual report says boys more worried by their appearance
The Children's Society says around 10,000 children across Wales are unhappy with their lives.
The charity's annual Good Childhood Report, which examines children's well-being in the UK, also reveals that children's happiness with life is at its lowest since 2009.
Across the UK as a whole, the charity claims almost a quarter of a million children report being unhappy with their lives.
It cites issues around boys' appearance, friendships and school as potential driving factors.
The charity is warning that the decline in happiness is a 'national scandal' and exposes huge failings for the country's most vulnerable children.
The report finds that nearly one in 12 boys (7.7%) aged 10-15 - the equivalent of 8,000 in Wales - are unhappy with their appearance. Over 13s were significantly less happy with how they look compared to 10-12 year-olds. Historically, boys have been happier with their appearance than girls but the gap is narrowing.
One boy said "Like Instagram and stuff, right? You see all these models, you see all these weight lifters, body-builders and you look at yourself and you're like - I look like a stick. I feel like we're exposed to a lot more so we are less secure about our appearance."
One in eight children (11.8%) are also unhappy with school, another key wellbeing measure, which is at its lowest since 2009. Evidence from year 10 students (mostly aged 14 and 15) suggested that bullying and not feeling safe at school are among a range of factors linked to low well-being. The research also found links between income poverty and financial strain and unhappiness with school.
A secondary school girl said: "A lot of things get stolen and people are rude and rough. Yesterday someone kicked my bag, called me a rude name and spat in my face."
Children's happiness with their friendships is decreasing with 2.8% of children unhappy with their friends. The wider research suggests that this could be down to factors as varied as bullying, being unable to spend time with friends outside of school, excessive social media use and loneliness.
The Good Childhood Report also found a third (33%) of 10-17 year olds have concerns about whether they will have enough money in the future, with more than a quarter (29%) worrying about having a job.
But children as young as 10 are also worrying about broad societal issues. The most common worries among 10 to 17 year olds were crime (42%), followed by the environment (41%) and information sharing online (37%). Only about a fifth worried about the economy and Brexit.
The Children's Society are calling on the Government to introduce national measurement of well-being for all children aged 11-18 to be undertaken through schools and colleges once a year. This would enable the experiences of young people to be recorded and issues acted upon for future generations.
Mark Russell, Chief Executive at The Children's Society says: "Modern childhood is a happy and carefree time for most, yet for too many it is not. It is a national scandal that children's unhappiness is increasing so quickly.
"Today's young people are becoming progressively unhappy with their friendships - one of the fundamental building blocks of well-being - as well as appearance and school.
"Children are also burdened with fears ranging from worrying about the future, not having enough money to not feeling safe at school and bullying. Many young people tell us they feel side-lined and ignored by those in power.
"The Children's Society is committed to listening to young people, and working with them to speak up and speak out, to enable every child to thrive not just survive.
"We are urging the Government to introduce a national measurement of children's well-being so we can really listen, respond and show young people they matter. Together we can build a brighter future and bring optimism and confidence back to being young."