THEY DO. Parents are legally responsible for much that their minor children doyou might want to consult a lawyer about this.
They also have ethical responsibilities for their children's actions, including the financial obligation you describe.
Talk to those parents. If that doesn't work, those with legal standing to represent the garden can sue the parents. Civil litigation is one way our society resolves such disagreements.
The community service assigned to the vandals is meant to deter them and others from ripping up radishes by their roots (at least other people's radishes) and to exemplify your community's values.
The financial restitution you seek from their parents is meant to restore the damaged garden to its original state.
Beyond their parental responsibilities, those families have duties simply as well-off members of the community. Such families have an obligation to support the community from which they gain so much.
(The New York Times Upfront, Vol. 142, March 15, 2010)