form with require if conditional validator

form.html

<h3>My children </h3>

<div class="form-check">
  <label class="form-check-label">
      <input class="form-check-input" type="checkbox" [(ngModel)]="hasChildren" name="hasChildren"> I do not have any children
    </label>
</div>

<div class="form-group">
 <label>Name</label>
 <input type="text" class="form-control" name="child1Name" 
         [(ngModel)]="child1Name"  
         [requiredIf]="!hasChildren" >
</div>

<div class="form-group">
 <label>birth Date</label>
 <input type="date"  class="form-control" name="child1Birthdate" 
        [(ngModel)]="child1Birthdate" 
        [requiredIf]="!!child1Name"> 
</div>