Police in England and Wales have taken DNA samples from children every 10 minutes, figures obtained using freedom of information requests suggest.
The Howard League for Penal Reform said officers taking 120,000 DNA swabs during 2010 and 2011 was "excessive".
Police said samples were taken from victims and witnesses and in paternity cases as well as from suspects.
But the Howard League argued that "overwhelmingly" the children were suspects, not victims.
'Up to mischief'
The Howard League, which submitted freedom of information requests to every police service in England and Wales, said officers took DNA swabs from almost 54,000 children in 2011, including swabs from at least 368 10-year-olds and 1,030 11-year-olds.
On average, officers therefore took samples from 27 primary school-aged children each week. Overall, the samples were mainly taken from boys, who accounted for about 70% of the swabs.
Howard League chief executive Frances Crook said: "When public money is tight and police forces are shrinking, it is disappointing to see valuable crime-fighting resources being wasted on taking DNA samples from thousands of innocent children while serious offences go undetected.
"Children who get into trouble with the police are usually just up to mischief."
The Association of Chief Police Officers (Acpo) said DNA could be taken from children in a number of circumstances.
A spokesman for Acpo added: "These may be when a child has been a victim of crime, when police would take DNA to confirm an incident took place and check whether it can be linked to a perpetrator.
"Others will be as part of criminal investigation where a child is the suspect. DNA samples are also taken to conduct criminal paternity tests as part of sexual offence investigations."
However, Ms Crook said: "Overwhelmingly the DNA was taken from children suspected of committing a crime, not victims. The argument by the police is that DNA collection is a valuable tool to catch serial rapists or other serious criminals, clearly irrelevant when dealing with children who have are suspected of stealing sweets from the local shop.
"It should be an extraordinary intervention for police to take the DNA from a child."
There is no official data to show the number of child arrests that have led to DNA tests.
However, a separate report from the Home Office shows that of a total of 7 million DNA swabs are currently being held across the UK for all age groups. Of these samples, only 39,450 were offered voluntarily.
The National DNA Database annual report for 2011 to 2012 also shows that 20% of the DNA samples currently in police possession were taken from people who were children at the time.
Under current law, police can keep the DNA of anyone they arrest for a recordable offence.
However, changes in the law outlining tighter restrictions on keeping DNA are due to come into effect later this year under the Protection of Freedoms Act.
For the first time, DNA samples of under-18s who are convicted of a first offence or minor offence will be deleted after five years.
Their DNA will be retained indefinitely only if they are convicted of a second offence or of a "qualifying offence" - a serious violent or sexual offence, terrorism offence or burglary offences.
The Home Office said: "Our changes will ensure DNA profiles are taken from the guilty and disposed of when people have done nothing wrong. Since this government came to power, more than 100,000 profiles taken from under-18s have already been deleted."
"DNA evidence is a crucial tool in the fight against crime, but it must be used proportionately."