The NSPCC said girls were six times more likely to be victims than boys
More than 21,000 sex offences against children were recorded last year - the equivalent of 60 every day, police figures obtained by the NSPCC suggest.
The children's charity said the figures, covering England and Wales, were "shocking", but the true extent of the problem was "far worse".
The figures have increased by 660 cases compared with the year before, although for 2007/2008 only 42 forces responded.
The Home Office said protecting children from sex abuse was a priority.
The statistics come out as the Home Office considers rolling out a pilot scheme which alerts parents to sex offenders.
The 2009 sex abuse figures were obtained through an NSPCC freedom of information request to all of England and Wales's 43 police forces. All forces responded.
Some of these children are so young they can't tell anyone what is happening
The statistics showed some 21,618 alleged sex crimes involving children were reported to police between April 2008 and March 2009. The figure was 20,758 between April 2007 and March 2008, but one force - Greater Manchester - did not provide any statistics.
One in seven victims were younger than 10, while 1,000 involved children aged five or under.
But Phillip Noyes, the NSPCC's director of strategy and development, said the figure was likely to be higher: "Some of these children are so young they can't tell anyone what is happening."
Girls were six times more likely to be victims than boys, the figures suggested.
The number of incidents where the offender knew the victim was four times higher than those involving strangers.
The Home Office compiles an annual crime report, which shows there were a total of 51,488 recorded sexual offences of all kinds between 2008 and 2009.
But the NSPCC said the official statistics do not provide a clear enough picture of how many involved children.
"Combining these statistics blurs the picture and even though detailed age breakdowns of victims are collected by police they are not passed to the Home Office," a NSPCC spokesman said.
The charity urged the government to publish the victims' ages and to "clearly link them with the number of convictions and other penalties resulting from the recorded offences".
"This information could then feed into a national sex abuse prevention strategy as well as help the development of local services to treat child victims," it said.
Barnardo's chief executive Martin Narey said the NSPCC had unveiled a "shocking reality".
"Barnardo's has worked with over 1,000 children in the past year who have been sexually exploited by a predatory adult. We agree that the true extent of this problem is not yet known," he said.
Karen Harvey, of Action for Children, said having more detailed information on the circumstances surrounding abuse - and the ages of children and young people - would help the charity develop more appropriate support services and effective preventative strategies.
"Demand for support far outstrips supply. Urgent funding must be made available to ensure access to vital therapies which can make a real difference to young survivors' lives," she said.
A spokesman for the Home Office said protecting children from sexual violence was a priority, which is why it had launched the child sexual offender disclosure pilot in 2008.
"Sarah's Law" - proposed after the murder of eight-year-old Sarah Payne by a convicted sex offender 10 years ago - is being tried out in Southampton, Warwickshire, Cambridgeshire and Stockton-on-Tees.
It allows parents to ask police if someone with access to their child has convictions or has been previously suspected of abuse.
The Home Office said interim results showed at least 10 children had been protected from potential abuse in the first six months.
"There are specialist child abuse investigation teams and specialist rape prosecutors in every area," a spokesman said.
"We are also committed to supporting the victims of sexual offences, and have invested £12m over the past five years in support services for children and adults.
"We collect figures by age on sexual violence in the British Crime Survey and we will continue to work with groups like the NSPCC to enhance our protection of the most vulnerable people in our society."
On Sunday Home Secretary Alan Johnson said early results of the child sex offender disclosure pilot were "extremely encouraging" but more analysis had to be conducted before a final decision about whether to extend the scheme nationally was made.
The so-called Megan's Law in the US, which allows the publication of names, addresses and pictures of paedophiles in some states, prompted calls for an equivalent "Sarah's Law" in the UK.