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);