Try to use your code in ngAfterViewInit event.
And you need to use parentNode property instead parent
export class App {
el: ElementRef;
constructor(el: ElementRef){
this.el = el;
},
ngAfterViewInit() {
const hostElem = this.el.nativeElement;
console.log(hostElem.children);
console.log(hostElem.parentNode);
}
}