More than 2,000 primary school-age children were arrested last year, an average of six a day.
The Howard League for Penal Reform said that 2,117 children aged 10 and 11 were detained in England and Wales in 2011.
Police arrested a total of 209,450 under-18s last year, down from 315,923 in 2008, the charity said. Around a fifth were girls.
The league has campaigned to reduce the number of child arrests on the grounds that they could lead to a criminal record for "being naughty".
Frances Crook, chief executive, said: "Children who get into trouble are more often than not just being challenging teenagers and how we respond to this nuisance behaviour could make a difference for the rest of their lives. An arrest can blight a life and lead to a criminal record for just being naughty. The positive change in policing children will release resources to deal with real crimes.
"Only a handful of children are involved in more serious incidents and they usually suffer from neglect, abuse or mental health issues. A commitment to public safety means treating them as vulnerable children and making sure they get the help they need to mature into law-abiding citizens."