@Component({
    selector: 'app-custom-paginator',
    template: '<mat-paginator #paginator pageSize="10"></mat-paginator>',
    })
export class CustomPaginator extends MatPaginator {
    @ViewChild(MatPaginator) paginator: MatPaginator;

    customMethod(): number {
        return 1
    }
}

@Component({
    selector: 'app-main-component',
    template: '<app-custom-paginator></app-custom-paginator>',
})
export class MainComponent implements OnInit, AfterViewInit {
    @ViewChild(CustomPaginator) paginator: CustomPaginator;
}