RouterModule.forRoot([
        {
            path: 'templates',
            component: TemplateLandingComponent,
            children: [
                {path: 'main-templates', component: MainTemplateComponent}
            ]
        },
    ])

<li><a [routerLink]="['/templates']" [routerLinkActive]="'active'">Templates</a></li>

<li><a [routerLink]="['/templates/main-templates']" [routerLinkActive]="'active'">Active Templates</a></li>