Benjamin said today's children were being exposed to "more and more cheap imports" and to "dross coming through the airwaves".
As a result, they had "no alternative" but to watch soaps and other programmes designed for adults, she said.
Benjamin, 59, called for a "balanced diet of programming" and for a return to "wholesome British-based" shows.
She blamed politicians for allowing commercial broadcasters to reduce their children's output and commercial "sharks" for venturing into the "waters of children's programming... to make a quick buck".
If politicians did not take action to fund public service content for children the genre could become a "graveyard", she said.
Benjamin told the Voice of the Listener and Viewer conference: "The UK has always had a strong tradition of delivering high quality children's programmes, not only on the BBC but also in the commercial sector.
"They were all essential viewing for all our children."
But she said: "Because many of these types of programmes are gone or are disappearing children have no alternative but to be sucked into adults programmes such as Deal or No Deal, or soaps being shown in the early evening.
"Children are being influenced by adult content before they have had a chance to develop."
Benjamin said parents always wanted to know "why our children are being exposed to more and more cheap imports which stifle our children's minds rather than stimulate them?"
She said: "The diet of programming children are receiving at the moment is sucking their brains.
"Children are being exposed to dross coming through the airwaves even in their own bedrooms.
"They are watching programmes which are totally unsuited to them on many levels. I would like to ban all televisions in children's bedrooms."
The BBC, which Benjamin said still delivered quality children's TV, once got "a run for its money" from commercial rivals with the likes of Thunderbirds, Rainbow, Press Gang, and Children's Ward.
Such programming had been banished to increase "daytime viewing figures and the profit for shareholders," Benjamin said.
Almost overnight "children's public service broadcasting at ITV was a thing of the past and there was nothing we could do legally", she said.
The former Play School presenter blamed politicians for failing to foresee the situation that would allow ITV to reduce its children's output, adding: "It's shameful that so little is thought about our children."
She added: "I believe we got to where we are because big business saw children's programming as a way of making millions.
"The sharks who had no real love for children's wellbeing ventured into our waters.
"This allowed commercialisation to spread like cancer into a once sacred enclave in broadcasting" and "decimated this once thriving industry".
"When advertising revenue dried up so did production and the death knell sounded even louder," she said.
The Prime Minister Gordon Brown had to "put measures in place now before it was too late".
Benjamin criticised the merchandising surrounding children's TV and called on broadcasters to put more ethnic minority faces on programmes for young people.
The children's presenter recounted making Coming to England for the BBC three years ago, a drama about a black girl arriving in Britain.
She said: "Children told me quite openly that they hated black people but after seeing Coming to England they realised they needed to get to know people before judging them.
"That is the power of television. Children's TV has a huge responsibility to fulfil these needs in a modern world filled with mistrust, commercialism, celebrity worship and the mad exploitation of young people.
"Children's television needs to reflect a diverse society, showing people old and young, that all cultures play an important part in our society."
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