You need to ensure that the MDI Child has a method called NewButton and that the method is declared as Public.
NewButton is a public Method. but i resoled it like this:
FormCollection fc = Application.OpenForms;
foreach (Form f in fc)
if (f.Name == "Frm_Supplier")
Exception Ex = new Exception();
by __avd: Added [code] tags. For easy readability, always wrap programming code within posts in [code] (code blocks).