The number of cases of peer-on-peer abuse in schools reported to a support charity has doubled in four years.
And the NSPCC, which has also seen a rise, says such cases are occurring "thousands of times a year".
Mosac, which helps parents and carers of abused children, says calls have risen from 27 in 2013 to 58 in 2017.
A mother who says her son was raped by a boy from school during a play date told Victoria Derbyshire even primary-age children were capable of sex abuse.
She said she wanted to make people aware and called for policy advice to help schools and social services cope.
Sarah (not her real name) had invited the boy, from her son's primary school, to their home when he suggested the boys "do something special with Lego" in her son's bedroom.
She said her son later told her he wedged a chair against the door handle and drew the curtains.
She said the boy, who was younger than her son, repeatedly asked to see her son's genitals, saying other boys always showed him theirs, and kept trying to pull his trousers and underpants down.
He allegedly said he would not be his friend if he didn't play with him, saying this was how he played with other boys.
Sarah said he then raped her son.
The boy told her son not to tell his parents, saying he would be angry with him if he did.
The abuse emerged at bedtime when Sarah's son explained why he had become so upset when the boy left – because someone had been hurting him.
When she asked whether the boy had done anything like this before, he replied that every time he had been to the boy's house, he had said the same things and attempted to rape him.
This happened on four separate visits, starting when he was eight.
Sarah said sexual abuse among young children was more common than people realised. She had not realised that some primary school-age children were physically capable of raping their peers.
Mosac, which supports non-abusive parents and carers of children who have been sexually abused, says it believes the scale of the problem is far greater.
"It's seismic. We should be really really concerned as a society," the charity's chief executive, Fiona Sim, said.
"Why on earth is this happening? What measures can we put in place? We must address this problem."
Of the 58 calls it received last year about child-on-child abuse, 19 related to children of primary-school age and 13 were of secondary-school age. Twenty-six were unknown.
The cases related to abuse both in and outside of the school environment.
The NSPCC has also said it has noticed an increase in calls about peer-on-peer sexual abuse – and many of the callers were now younger.
It is lobbying the government to launch a study to assess the scale of sexual abuse of children, to include this kind of abuse.
Jon Brown from the charity, said society had to wake up to the fact that this was happening across the UK thousands of times each year.
"Tackling this problem demands all children are introduced to key learning concepts, such as boundaries and consent, from primary school onwards," he said.
After Sarah's son told her what had happened she called the NSPCC support line, which advised her to contact the school's safeguarding officer and to call the police.
The mother said the head teacher was stunned and sympathetic about why she did not want to send her son back to school.
Sarah says she believed the school subsequently introduced safeguarding measures, especially around the perpetrator, and that local child-protection services were also contacted.
Last December the Department for Education released interim guidance to give schools a framework to form individual policies on sexual violence and harassment in schools and colleges.
But Sarah says the advice did not properly address primary schools – where the children are below the age of criminal responsibility and need to be treated differently.
She is also concerned about what measures are taken to safeguard children interacting with alleged perpetrators at after-school clubs, play dates, parties or locally run groups and clubs.
And Mosac says professionals who don't know what to do or where to get help have also been in touch.
A specialist police officer spoke to their son but advised that because the other boy was under the age of criminal responsibility – 10 – no charges would be brought.
The case was referred to social services and her son was awarded compensation from the Criminal Injuries Compensation Authority after his case was reviewed.
Sarah says her son has suffered. "He's not the same boy. He's still regularly prone to low moods," she said.
"He's had two rounds of counselling, which has helped. Thankfully he's now made new friends – but he's the one who has had to start afresh."
Sarah said society needed to talk about this kind of abuse.
"Our society's approach to this is hiding away – how do we/should we react? I shouldn't be ashamed, yet it is shameful and embarrassing to talk about – I feel to blame despite having done nothing wrong."
A Department for Education spokesperson said: "Pupils and parents rightly expect schools to be safe places where children are protected from all kinds of abuse. That's why we are clear that all schools must have a child protection policy in place that addresses abuse between pupils.
"We know that these cases can be incredibly complex and sensitive and following feedback from parents and schools, we recently published updated advice to help schools prevent and respond to incidents of sexual violence and sexual harassment between children."
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