Users without email


I want to create blog users without an email address (think of a child who does not have its own email). Unfortunately, when creating users in WPMU, giving a valid email address is always required (probably because of the recover password function). I would give those users the email of the admin (think of the father's email for his child), but WPMU won't accept (two users must not have the same email). Any idea how I can allow users to not have an email address or to share the email with some other user? (I don't care about the recover password function.)