mdi child Form question

[gfaryd]

in my application i have main form whose mdicontainer property is true
i open aform as its child form by typing

Form frmchild=new Form();
frmchild.MDIParent=this;
frmchild.Show();

from this form i open another form. how can i make this new form as child form of main form which is mdi container

Form frmchild_new=new Form();
frmchild_new.MDIParent=???????/;
frmchild_new.Show();

Regards,
gfaryd