Hi, 

you can do this from the DB directly. Do you have access to the database?

Execute:
    
    UPDATE res_partner SET active = 'f' WHERE parent_id in (SELECT id FROM res_partner WHERE is_company = 't' AND active = 'f')

Also, you can create a function on_change for the active field of a partner like this:

    def onchange_active_partner(self, cr, uid, ids, active, context=None):
        if not active:
            childs_ids = self.search(cr, uid, [('parent_id','in',ids)])
            self.write(cr, uid, childs_ids, {'active': False})
        return {'value' : {}, }

Hope this helps!

UPDATE:

You will have to create a new file (a good name will be res_partner.py)

# -*- coding: utf-8 -*-

from openerp.osv import osv, fields

class res_partner(osv.osv):
    _inherit = 'res.partner'

        def onchange_active_partner(self, cr, uid, ids, active, context=None):
            if not active:
                childs_ids = self.search(cr, uid, [('parent_id','in',ids)])
                self.write(cr, uid, childs_ids, {'active': False})
            return {'value' : {}, }

res_partner()
# vim:expandtab:smartindent:tabstop=4:softtabstop=4:shiftwidth=4:

And put it into the __init__.py file like
import res_partner

Finally add the onchange function to the views:

<?xml version="1.0" encoding="UTF-8"?>
<openerp>
    <data>
        <record id="view_partner_form_onchange_inherit" model="ir.ui.view">
            <field name="name">res.partner.with.onchange.form.inherit</field>
            <field name="model">res.partner</field>
            <field name="inherit_id" ref="base.view_partner_form"/>
            <field name="arch" type="xml">
                <field name="active" position="attributes" >
                  <attribute name="on_change">onchange_active_partner(active)</attribute>
                </field>
            </field>
        </record>
    </data>
</openerp>