foreach (Form childForm in MdiChildren)
            {
                childForm.Close();
            }
            Form2 obj= new Form2();
            obj.MdiParent = this;
            obj.Show();

if you opening the child form on button click event FROM parent window....

then..... use following Code Snippet......

     

   // Create a new form to represent the child form
   frmChild frm = new frmChild();

   // Make the new form a child form.
   frm.MdiParent = this;

   // Display the child form.
   frm.Show();

from parent

child.show(this);