The NSPCC says many young children are unable to speak out
More than 20,000 sex offences against children were recorded last year - the equivalent of 50 every day, police figures obtained by the NSPCC suggest.
But the children's charity says the "grim" figures, covering England and Wales, show just a "snapshot" of suffering endured by under-18s.
It is calling on the government to publish annual police data showing the accurate number and ages of victims.
The Home Office said it was committed to bringing sex offenders to justice.
It's a grim picture but this is only a snapshot... there are many more abused children whose suffering never comes to light
The 2008 sex abuse figures were obtained through an NSPCC freedom of information request to all of England and Wales's 43 police forces. Only Greater Manchester Police did not respond.
They showed some 20,758 alleged sex crimes involving children were reported to police last year.
In 4,984 cases - almost one in four - the victims were aged 10 or younger, while 800 involved pre-school children.
Girls were six times more likely to be victims than boys, the figures suggested.
The Home Office compiles an annual crime report, which shows there were a total of 53,540 recorded sexual offences of all kinds between 2007 and 2008.
But the NSPCC says the official statistics do not provide a clear enough picture of how many involved children.
"It's a grim picture but this is only a snapshot, as our research indicates there are many more abused children whose suffering never comes to light," said NSPCC director of public policy Phillip Noyes.
"Clearly, very young children who are sexually abused can't speak out and it's currently very difficult to get a clear understanding of how many child sex abuse victims there are.
"If we are able to get these details every year it will start to build a more accurate picture of what is happening and we can make more concerted efforts to protect children."
Shaun Kelly, head of safeguarding at the charity Action for Children, described the figures as "uncomfortable reading".
Although Action for Children worked with more than 178,000 vulnerable children access was "a postcode lottery", he said.
"More funding must be made available to ensure those who need help are able to receive it, especially for children and young people who find it the hardest to speak out," he added.
A spokeswoman for the Home Office said the government was committed to increasing the number of sex offenders caught and brought to justice and that support services had been improved.
"We now have specialist child abuse investigation teams and specialist rape prosecutors in every area and provide operational support to forces to help improve their strategies for the investigation of serious sexual offences," she said.
"We are also committed to supporting the victims of sexual offences, and have invested Ŗ11.25m over the past five years in support services for children and adults.
"We look forward to discussing the NSPCC's suggestions with them."