The children's commissioner for England says OnlyFans needs to do more to stop underage users after the BBC revealed some were selling explicit videos.
BBC News found under-18s have used fake identification to set up accounts on the site and police say a 14-year-old used her grandmother's passport.
Dame Rachel de Souza has written to the British website expressing her "shock" and "deepest concern" at the findings.
OnlyFans said it plans to meet her and correct any misinformation.
The social media site allows people to share videos, photos and messages directly with subscribers for tips or a monthly fee.
In return for hosting the material, OnlyFans takes a 20% share of all payments.
Founded by Essex businessman Tim Stokely in 2016, the site has boomed during the pandemic and now has more than 120 million users.
Creators can post a range of content from cooking to fitness videos, but it is best known for pornography.
But BBC News found that under-18s have sold explicit videos on the site, despite it being illegal for people to share indecent images of children.
This included a 17-year-old who sold explicit videos of herself masturbating and playing with sex toys after signing up with a fake driving licence.
While a fake ID did not work, BBC News was also able to set up an account for an underage creator by using her 26-year-old sister's identification, showing how the site's age verification process could be cheated.
Now the Children's Commissioner for England has told BBC News the site needs to do more to tackle underage users illegally selling explicit images on the platform.
"Children paid a huge price for keeping us all safe during the pandemic I think it's time we gave them something back," said Dame Rachel. "On any level no child should need or want to sell images of themselves."
In a letter to Mr Stokely, seen by BBC News, Dame Rachel said she wanted to discuss what the company is doing to "eradicate the harm that children are experiencing" on the site.
"You say that you have replaced your age verification system with an 'exceptionally effective' one, yet it appears to have been easily circumvented by the BBC team," it adds. "This is unacceptable."
In a statement, OnlyFans said it had received the letter and is planning a meeting with Dame Rachel "to explain our trust and safety processes and correct any misinformation".
The company added: "We take our obligations extremely seriously."
In response to the BBC News investigation, OnlyFans said it had closed the accounts flagged and refunded all active subscriptions.
It added that its age verification systems exceed regulatory requirements and continue to evolve as new technology becomes available to "help us to reduce incidences of under-18-year-olds becoming OnlyFans users".
Have you had an experience on OnlyFans? Please share your story by emailing firstname.lastname@example.org.
Please include a contact number if you are willing to speak to a BBC journalist. You can also get in touch in the following ways: