Sussex Police took DNA from 1,290 children during 2011, research claims.
Officers took swabs from boys and girls aged 17 or under, the Howard League for Penal Reform found.
This included 19 primary school-age children, three 10-year-olds and 16 11-year-olds.
Many of the children required to give a sample will not have been charged with a criminal offence, the Howard League claimed.
Chief Executive Frances Crook said: “Children who get into trouble with the police are usually just up to mischief. Treating so many like hardened criminals by taking their DNA seems excessive.”
Sussex Police said the number of DNA samples taken from children fell dramatically in 2011 compared to 2010 as a result of fewer arrests of children. A spokesman said: “We are aware that children’s behaviour can range from childish through to criminal, and ensure that we are proportionate in our dealings with them.
“We carry out significant prevention work with partner agencies in communities, within schools and youth offending teams (YOTs) This is aimed at preventing offending by young people and continuing to reduce first time entrants to the criminal justice system.”