Bring back programmes such as sure start. Give parents support right at the start.
Bring back health visitors to how they used to be, who were great at sign posting to services for help and support.
Give women more help to escape domestic violence, usually perpetuated by men, meaning children don't have to grow up in violent households.
A better system of maintenance from absent fathers, meaning fewer children grow up in poverty.
Better mental health support, accessible in the early stages for children and adults rather than waiting until the shit hits the fan.
Prisons having a focus on rehabilitation rather than prisoners just being caged up.
More funding for schools to support children right from reception class.
More youth services that can help steer kids away from gangs.
Drug and alcohol support availability much sooner than currently. (Can take a long time to get a detox and rehab place)
There's loads that could be done that could help prevent violence in general. The above is just the tip of the iceberg.