gistfile1.txt

UPDATE

createForm() {
    this.frm = this.fb.group({
      delivery: this.delivery,
      invoice: this.invoice
    });
    this.frm.get('invoice').disable();
  }
  show() {
    this.hasShowed = !this.hasShowed;
    if (this.hasShowed) {
      this.frm.get('invoice').enable();
    } else {
      this.frm.get('invoice').disable();
    }
  }


export class ParentFormComponent implements OnInit {
  frm: FormGroup;
  delivery: FormGroup = this.fb.group({
    name: '',
    email: ''
  });
  invoice: FormGroup = this.fb.group({
    name: '',
    care: ''
  });
  hasShowed = false;
  constructor(private fb: FormBuilder) {}

  ngOnInit() {
    this.createForm();
  }
  createForm() {
    this.frm = this.fb.group({ delivery: this.delivery });
  }
  show() {
    this.hasShowed = !this.hasShowed;
    if (this.hasShowed) {
      this.frm = this.fb.group({
        delivery: this.delivery,
        invoice: this.invoice
      });
    } else {
      this.frm = this.fb.group({ delivery: this.delivery });
    }
  }
  onSubmit() {
    console.log('Submit:', this.frm.value);
  }
}

<form [formGroup]="frm" (ngSubmit)="onSubmit()">
  <app-delivery [parent]="frm"></app-delivery>
  <app-invoice *ngIf="hasShowed" [parent]="frm"></app-invoice>
  <button type="submit" class="btn btn-primary">Send</button>
</form>
<button type="button" class="btn btn-secondary" (click)="show()">
  show
</button>