this.$state.go('parent', {params: this.$state.params.PARAMS});

Parent state declaration:
export const parentState: Ng2StateDeclaration = {
  name: 'parent',
  url: '/parent',
  views: {
    content: {component: ParentComponent}
  },
  params: {
   // How to get CODE param from child to here ??
  }
};

Child state declaration:
export const childState: Ng2StateDeclaration = {
  name: 'parent.child',
  url: '/child/:CODE',
  redirectTo: 'parent',
}