Social media sites should be fined if they fail to protect children from harmful content, according to the NSPCC.
Facebook, Twitter and Askfm are among such firms which ought to be under "robust scrutiny" from a new watchdog ensuring they shield youngsters from pornography, violent content, child abusers and online bullying.
Peter Wanless, the head of the children's charity, urged the issue to be made a priority by the next government.
Just as films and television are regulated, so too should sites where many children spend hours browsing and communicating, he said.
His demands come in the wake of an NSPCC survey which found four in five youngsters felt social media companies needed to do more to protect them while using their services.
In a letter to The Times, he said it was time for the Government to take "bold action to hold internet companies to account" - calling for the installation of a new watchdog "with bite".
"Online safety is one of the biggest issues for children and young people today and one that the Government must tackle head on," he wrote.
"It is high time for online companies to come under robust scrutiny from an independent regulator with bite and to face fines when they fail to keep children safe."
Mr Wanless also called on the sites to offer specially designed accounts to protect youngsters with default privacy settings, content filters and guards against "groomers".
On Thursday MPs passed the Digital Economy Bill, which includes a code of conduct for social media giants to tackle illicit material.
The Government is reportedly considering fining companies who do not comply with the code.
Mr Wanless added: "We already protect children from viewing inappropriate or violent content at the cinema and on television.
"Given that today's children spend their free time online, why do we not afford them the same protections in this sphere?"