Almost 9,000 children were arrested by West police last year – more than 170 every week, the world's oldest penal reform organisation has found.
But figures from the two forces in the region also show a steady decline over the past four years, according to the Howard League for Penal Reform.
Avon and Somerset Police made 5,608 arrests of boys and girls aged 17 and under during 2011 and in Devon and Cornwall the figure was 3,363.
The overall drop from 14,664 in 2008 to 8,971 in 2011 was around 39%, a trend the League says it hopes will continue, allowing officers to focus vital resources to deal with "real crimes".
Frances Crook, chief executive of the Howard League for Penal Reform, which was founded in 1866, 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.
"Only a handful of children are involved in more serious incidents and they usually suffer from neglect, abuse or mental health issues."
More than one million child arrests have been made in England and Wales since 2008, but the figures show a downward trend. The number of arrests nationwide fell by a third between 2008 and 2011.
Across England and Wales, police made more than 209,000 arrests of boys and girls aged 17 and under last year.
Girls account for about a fifth of arrests each year, 207,808 between 2008 and 2011.
However, 24,055 fewer girls were arrested in 2011 than in 2008 – a fall of 38 per cent.
The Howard League claimed the reduction was "testament to a change of culture" with officers more focused on public safety than targets.
Avon and Somerset Police said the introduction of restorative justice had helped reduce arrests.
Inspector Paul Cox, who has responsibility for youth issues, said: "Once a young person enters the criminal justice system they are stigmatised and more likely to re-offend. Restorative justice gave us another tool to work with young offenders, giving the opportunity to give them a warning and a second-chance on their first offence but whilst also making them face the consequences of what they had done."
Devon and Cornwall Police said its officers did not make the decision to detain youngsters "lightly".
A spokesman said: "There are instances where it is necessary to arrest a younger person to prevent or investigate crime or to stop an individual putting themselves or other members of the public at risk of harm.
"All police forces use out-of-court disposals to deal with offences where appropriate and these figures do show a declining trend in the number of young people girls being arrested and brought into the criminal justice system.
"But in certain circumstances taking them into custody may can be the only option available."
