app.component.ts

import { Component, ViewChild } from '@angular/core';
import { SomeChildComponent } from './someChild/someChild.component';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})
export class AppComponent {
  @ViewChild(SomeChildComponent) someChildComponent;

  callChildFunction() {
    console.log(this.someChildComponent); // Here you have one reference to your child component

    this.someChildComponent.callSomeFunction();
  }
}