My helixviewport3d is in the mainwindow. And I want to add children from usercontrol to helixviewport3d in mainwindow in C# ?

My HelixViewport3D is in the mainwindow. And I want to add children from UserControl to HelixViewport3D in MainWindow in WPF C# so please help me 

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();

        // Get the UserControl instance
        UserControl1 uc = (UserControl1)MainGrid.FindName("MyUserControl");

        // Add the UserControl to the HelixViewport3D as a child
        helixViewport3D.Children.Add(uc);
    }
}