Alarmingly 44 per cent of girls aged between seven and 10 said strangers have messaged them or sent them friend requests while they were playing games online.
The issue of being contacted online by strangers appeared to have grown, with 30 per cent of this age group saying this has happened them, compared with 16 per cent in 2016.
Less than a fifth of all those who took part described themselves as very happy, down from 40 per cent of respondents in 2009 when the Girls’ Attitudes Survey began.
Researchers said the sharpest decline in happiness was among the youngest girls – those aged between seven and 10 years old - at just 28 per cent when asked this year compared with 57 per cent of those asked in 2009.
A total of 2,614 girls and young women aged between seven and 21 took part in this year’s research between February and April.
The number of 13 to 21-year-olds who have received sexist comments online has more than doubled since 2018.
Four in 10 of those aged 11 to 21 said they had been shouted at or whistled at in the street on the way to or from school, the survey’s section on sexual harassment found.
More than two thirds (69 per cent) of 11 to 16-year-old girls said boys at their school had made comments about girls and women that they would describe as “toxic”, while 44 per cent said male pupils had made comments about girls and women that have made them feel scared for their safety.
When it came to appearance, more than two-thirds of girls aged between 11 and 21 said they sometimes feel ashamed of the way they look because they are not like girls and women they see in the media and online.
Just over a third of those aged between 11 and 21 said they would consider some form of cosmetic surgery, a rise from 29 per cent in 2018, with almost a quarter in this age group this year saying they would consider various procedures after having seen images of influencers or celebrities getting them done.
Girls and young women also revealed their fears for the future, with 48 per cent of those aged 11-16 saying they feel it will be more difficult for them to get a job when they leave education than it was for young people five years ago.
Childline said the Girlguiding survey findings were “sobering and echo the concerns that our trained Childline counsellors hear on a daily basis”.
Kieran Lyons, from the charity, said: “Every year, thousands of girls contact the service about issues that are affecting their mental health and wellbeing including pressures around their appearance, as well as harassment and abuse that they face online and in the classroom.”
A Government spokesperson said: “All children deserve to grow up in a safe environment and the Government is committed to supporting women and girls to feel safe everywhere.
“It is unacceptable that women and girls suffer disproportionately from abuse online and the Online Safety Bill takes important steps to address this.
“Through the mandatory Relationship, Sex and Health Education curriculum all pupils learn about respectful relationships and that everyday sexism, misogyny and gender stereotyping are not tolerated.”