class ModelX(models.Model):

    flag_childs = fields.Char('Label XXX', compute='_compute_flag_childs')

@api.depends('child_ids')

    def _compute_flag_childs(self):

        for record in self:

             record.flag_childs = record.child_ids.ids and len(record.child_ids.ids) or ''

This old link can help any body facing same problem. \https://code.launchpad.net/~therp-nl/openerp-web/7.0-lp1013636-x2m_honour_required_attribute

You can add method similar like this (example in idea module):

def _check_name(self,cr,uid,ids):
    for idea in self.browse(cr, uid, ids):
       if 'spam' in idea.name: return False # Can't create ideas with spam!
           return True

...but instead would check the coefficient_ids field that something was passed.

Then add to constraints (for validation):

  _constraints = [(_check_name, 'Please avoid spam in ideas !', ['name'])]

Update:

This would be validation for other field types, I'm not exactly sure for one2many or many2one validation. I believe this maybe possible only for many2one.