Angular 4 / Reactive Forms - Add Control in a child component

I have a parent component with a FormGroup. In my html code, I call several times a child component called my-text-input for the exemple. In this child component, I have one Input text with a specific attribut FormControlName. I want to add this control to my main form.

My main component with the formGroup :

<form class="form-horizontal" [formGroup]="mainForm" (ngSubmit)="save()" 
novalidate>
 <div class="devis-container">
 ...
<!-- first call of my child component -->
<my-input-text [parent]="mainForm" [model]="myObject.name" class="size-xl" 
[required]="true"></my-input-text>
...
<!-- and in another part of the main form, another declaration -->
<my-input-text [parent]="mainForm" [model]="myObject.surname" class="size-
xl" [required]="true"></my-input-text>
...
</div>
</form>

The HTML of my child component :

<form [formGroup]="parent" name="inputTextForm">
    <input type="text" formControlName="textField" placeholder="{{placeholder}}" class="domia-input"/>
    ...    
</form>

So my issue is that I don't know how to add my control 'textField' to my main form. I tried this :

export class MyInputTextComponent extends InputBaseComponent implements OnInit {
    ...
    @Input()
    parent: FormGroup;
    ...
    ngOnInit() {
     this.parent.addControl('textField', 
        new FormControl('', 
            Validators.compose([conditionalRequired(this.required), 
            Validators.pattern(this.regex), 
            Validators.minLength(this.minlength), 
            Validators.maxLength(this.maxlength)]))            
        );
    this.parent.controls.textField.setValue(this.model);
    }
    ...

But the values of two "my input text" are the same. I found it is logical because it is the same form control I think. So, I tried another solution in the OnInit method of my child component :

this.parent = this.formBuilder.group({ 
    textField: [this.model, conditionalRequired(this.required)]
});

In this case, the values are ok but I don't see this control in my mainForm. I think it is logical because I don't add my control to my parent/main form.

I tried to find some example on the official documentation page but I didn't find this specific case. Another solution should be to declare the control textField in the method BuildForm of my main component but I would like to find another solution and declare the control in the child component.

Thank you for your help.