The mother of murdered teenager Brianna Ghey has called for "drastic action" to protect children after rejecting Rishi Sunak's insistence the Online Safety Act was adequate.
On Sunday Esther Ghey said children must be stopped from having access to social media apps.
She has now reiterated the call after the prime minister said the new law alone would be robust enough.
Brianna, 16, was stabbed to death after being lured to a park in Cheshire.
Her teenage killers - Scarlett Jenkinson and Eddie Ratcliffe - were given life sentences on Friday.
The pair, both 15 at the time of the murder, had a fascination with violence, torture and murder and had planned the killing for weeks using a messaging app.
Jenkinson had also watched videos of violence and torture on the dark web.
Ms Ghey is campaigning for under-16s to be blocked from accessing social media on smartphones and stronger parental controls to flag potentially harmful searches to parents.
Asked what changes she would like to see made in the smartphone industry, Ms Ghey told OceanNewsUK: "We'd like to see mobile phone companies take more responsibility.
"I would like to see the law change so that children only have access to children's mobile phones, and that could look exactly the same as an adult's mobile phone but without the ability to download social media apps, and there is software available already.
"Schools use it and we could link it up to a parent's phone and if any words are being searched like the words that were searched during the run-up [to Brianna's murder]... it could be flagged up on a parent's phone, and then parents are aware of any concerning things that children are looking at."
Ms Ghey said she thought the Online Safety Bill was "a step in the right direction" but would not be enough on its own.
She added: "I think that we do need something a little bit more drastic for children."
Asked whether a mobile phone was the cause of what happened to Brianna, Ms Ghey said: "I'm not sure whether it started it all, but it definitely didn't help.
"I think that the content that they [her killers] were looking at online, it probably fed what was already there."
Speaking to OceanNewsUK, Prime Minister Rishi Sunak insisted he believed the Online Safety Act was robust enough to protect children online.
He said the new law gave Ofcom the power "to actually talk to social media companies and ensure that children aren't exposed to harmful content online".
