Longfield warned that children "remain an afterthought" for some of the best-known social media firms and she has already penned a letter to the companies - including YouTube, Instagram, Pinterest and Facebook - calling on them to address the issue of disturbing content.
She wrote the letter following the recent suicide of 14-year-old Molly Russell, who took her own life after she watched self-harm images on Instagram.
The letter reads: "The tragic suicide of Molly Russell and her father's appalled response to the material she was viewing on social media before her death have again highlighted the horrific amount of disturbing content that children are accessing online.
"I do not think it is going too far to question whether even you, the owners, any longer have any control over their [platforms'] content.
"If that is the case, then children should not be accessing your services at all and parents should be aware that the idea of any authority overseeing algorithms and content is a mirage."
In response to the letter, the social media platforms have insisted they are determined to keep their sites safe.