WPF Setting focus to UserControl added at runtime

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WPF

I know there has been much discussion on this subject but I'm afraid I do not get it. I have a Parent UserControl to which I am adding a Child UserControl at runtime. The Child UserControl has a menu. The Menu items are not enabled until the Child UserControl has keyboard focus. The Child UserControl is added with the click event of a button.

How do I set the focus to the Child UserControl?

I have tried all sorts of variations of Focus() and KeyBoard.Focus to no avail. The focus remains on the clicked button of the Parent UserControl.

class ParentUserControl
{
    public void ChangeContent(UIElement newContent)
    {
        var userControl = newContent as IBaseUserControl;
        if (userControl == null)
        {
            throw new NoNullAllowedException();
        }

        if (GridControl.Children.Count == 0)
        {
            GridControl.Children.Add(newContent);
            userControl.GetObservableCollection();
        }
        else
        {
            GridControl.IsHitTestVisible = false;
            var oldContent = GridControl.Children[0];

            EventHandler onAnimationCompletedHandler = delegate
            {
                GridControl.IsHitTestVisible = true;
                GridControl.Children.Remove(oldContent);
                userControl.GetObservableCollection();
            };

            FadeAnimation(newContent, oldContent, onAnimationCompletedHandler);
        }

        // None of these work
        //GridControl.Focus();
        //newContent.Focus();
        //Keyboard.Focus(newContent);
        Keyboard.Focus(GridControl);
    }

}