How to add a MDI Child from another MDIChild

You should set the MDIParent of the new child form as follows:

private void buttonAdv1_Click(object sender, System.EventArgs e) 

{ 

Form3 child1 = new Form3(); 

// Set the new form's MDIParent to the main form. 

child1.MdiParent = (Form1) this.Parent.Parent; 

child1.Text = "Document"; 

child1.Show(); 

}
Private Sub buttonAdv1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles buttonAdv1.Click

Dim f1 As Form3 = New Form3()

' Set the new form's MDIParent to the main form. 

f1.MdiParent = CType(Me.Parent.Parent, Form1)

f1.Text = "Document"

f1.Show()

End Sub