Im Using openerp v7, presently i'm displaying the parent and child grid structure separately like

parent_Id1
parent_Id2
parent_Id3
child_id1
child_id2
child_id3

Please find the below code as using:

<field name="Parent_Id">
 <tree string="Purchase Order Lines">
    <field name="product_id" />
    <field name="name"/>
    <field name="slab_qty"/>
    <field name="date_planned"/>
    <field name="product_qty" />
    <field name="price_unit"/>
    <field name="taxes_id" />
    <field name="price_subtotal"/>
    <field name="Child_id">    
 </tree>
</field>

<field name="Child_id">                         
 <tree string="Child details" >
   <field name="product_id"/>
   <field name="s_no"/>
   <field name="Listprice" />
   <field name="unitprice" />
 </tree>
</field>

I want to display the grid/tree structure like this

parent_Id1
 child_id1
parent_Id2
 child_id2
parent_Id3
 child_id3

Is any possibility in openerp to make the parent or child based tree structure. Help me to achieve this?