Angular 2+ conditional validation with reactive form

<form [formGroup]="userForm" (ngSubmit)="onSubmit()" novalidate>
		
    First Name:
    <input type="text" formControlName="first_name"/>
    <span *ngIf="userForm.controls.first_name.touched && userForm.controls.first_name.errors?.required">
      Please first name.!
    </span>
    <br>

    Last Name:
    <input type="text" formControlName="last_name"/>
      <span *ngIf="userForm.controls.last_name.touched && userForm.controls.last_name.errors?.required">
      Please enter last name.!
    </span>
    <br>

    Do you have child?
    <input type="checkbox" (change)="isChildChange($event)" formControlName="isAnyChild"/>
    <br>
    <ng-container *ngIf="userForm.value.isAnyChild">
      Child Name:
      <input type="text" formControlName="child_name"/>
        <span *ngIf="userForm.controls.child_name.touched && userForm.controls.child_name.errors?.required">
        Please enter child name.!
      </span>
    </ng-container>
    <br>
  <input type="submit" [disabled]="!userForm.valid"/>
</form>

import { Component } from '@angular/core';
import { FormBuilder, FormGroup, FormControl, Validators, FormArray } from '@angular/forms';

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: [ './app.component.css' ]
})
export class AppComponent  {
  userForm: FormGroup;
  constructor() {
    this.userForm = new FormGroup({
      first_name: new FormControl('', [Validators.required]),
      last_name: new FormControl('', [Validators.required]),
      isAnyChild: new FormControl('',),
      child_name: new FormControl('')
    });
  }

  onSubmit() {
    if(this.userForm.valid) {
      console.log(this._v());
    }
  }
  _v() {
    return this.userForm.value;
  }
  setRequired() {
		return [Validators.required];
	}
  isChildChange(event) {
    if(this.userForm.value.isAnyChild) {
      this.userForm.controls.child_name.setValidators(this.setRequired());
    } else {
      this.userForm.controls.child_name.clearValidators();
    }
  }
}