Dyfed Powys Police recorded the most crime committed by under 10s of any police force in Wales.
Data from a Freedom of Information request, showed the number of serious crimes committed by children under the age of 10 has risen.
Incidents include rape and violent assault.
Throughout Wales, 309 crimes were committed in 2015/16, compared to 297 the previous year, a four per cent rise.
Three of Wales’ four forces reported a significant upsurge in crime, with Dyfed Powys recording the most significant rise.
The force recorded 123 – this compared to 104 crimes recorded the previous year, representing an 18 per cent rise.
Where an offence is committed by someone under 10 years old they cannot be charged with committing a criminal offence as they have yet to reach the age of criminal responsibility.
However all children below 10 years old who commit a crime are subject to child protection referrals, so that joint multi-agency interventions can be undertaken, to prevent future offending and safeguard the child offenders and any others at risk of their behaviour.
In a two-year period, crimes included 88 acts of violence against a person, 52 criminal damage offences and 38 sexual offences.
A spokesperson for Dyfed Powys Police said: “In line with Home Office guidelines, Dyfed Powys Police records all crimes committed by under 10s, so as to show a true picture of the issues.
“Also School Community Police Officers across the force area deliver targeted age appropriate lessons in schools as part of the All Wales School Liaison Core Programme.”