Solved, even if  with a not so elegant, pythonic way.

On pos.adm I created a new def since is quite easy search for the correct one (pos_session.name =is equal to pos_adm.pos_ref):

@api.multi
def _update_tree(self):

    pse = self.env['pos.session'].search([('name', '=', self.pos_ref)])
    pse.write({'order_paid': pse.order_paid + 1})

Since order_paid field is just an index used to trigger function is not a problem use it to force a db check.

After that I simply wrote some lines of xml for a function called by a button.

    <record id = "update" model = "ir.actions.server">
        <field name = "sequence" eval = "5"/>
        <field name = "name">update_session</field>
        <field name = "model_id" ref = "model_pos_adm"/>
        <field name = "condition">True</field>
        <field name = "type">ir.actions.server</field>
        <field name = "state">code</field>
        <field name = "code">
pos_obj = env['pos.adm'].browse(context.get('active_id'))
pos_obj._update_tree()
        </field>
    </record>

The button call:

<button name="%(update)d" string="Up" type="action"
        class="oe_highlight" attrs = "{'invisible':[('state','!=','opened')]}"/>

Now I have a quicker real time update on my tree view.