Blend Visual Studio : how to add childrens to an UserControl?

Question

I am using Blend Visual Studio 2019, I have followed this post -https://stackoverflow.com/questions/9094486/adding-children-to-usercontrol

But I get this error:

Cannot set Name attribute value 'TextDateOrder' on element 'TextBlock'. 'TextBlock' is under the scope of element 'Box', which already had a name registered when it was defined in another scope. Line 14 Position 24.

I do understand the error, but do not know how to fix it.

How do I fix my error?

[ContentProperty(nameof(Children))]
public partial class Box : UserControl
{
   public static readonly DependencyPropertyKey ChildrenProperty = 
            DependencyProperty.RegisterReadOnly(nameof(Children), typeof(UIElementCollection), typeof(Box), new PropertyMetadata());

   public UIElementCollection Children
   {
       get { return (UIElementCollection)GetValue(ChildrenProperty.DependencyProperty); }
       private set { SetValue(ChildrenProperty, value); }
   }

   public Box()
   {
       InitializeComponent();
       Children = PART_Host.Children;
   }
}