Show children forms in modal dialog

Hi,

I have a form with the property 'name' set to 'frmMain' and the property 'isMdiContainer' set to 'true'.

After, i have another form with the property 'name' set to 'frmChild' and the property 'MdiParent' set to 'frmMain'.

I want to show the form 'frmChild' in modal dialog with the instruction 'frm.ShowDialog()'.

However, to execute that instruction, visual basic shows this error:

'A form that is not top level can not be displayed as a modal dialog box'

Is there any way to display in modal dialog a form that is a child of another?