In Odoo Menu-Item parent-id is required.
If you not give parent menu item then odoo will consider menu item as Parent Menu item.
You have done same thing.create menu item without parent-id,so odoo will consider as parent menu.
Ex:
Sales Is parent Menu. ( In base module)
<menuitem name="Sales"
id="menu_base_partner"
groups="base.group_sale_salesman"
sequence="20"/>
Product is Child Menu.
<menuitem id="menu_product" name="Products" parent="base.menu_base_partner" action="product_action_window" sequence="9"/>
Parent Menu item is base.menu_base_partner.
Child Menu item is menu_product.
If you create only Parent menu item then Child menu item will not visible in the system.