Class to minimize all forms

Class to minimize all forms

How I will Convert this into class which minimize all the childform? When
I try to transfer it into class I got error :
The type or namespace name 'MdiChildren' could not be found (are you
missing a using directive or an assembly reference?)
public void minimizeAll()
{
foreach (Form childForm in MdiChildren)
{
childForm.WindowState = FormWindowState.Minimized;
}
}