AngularJS 1.5.x

var app = angular.module('plunker', []);



app.component('child', {
  template: `
    <h2>Child</h2>
    <p>Message: {{$ctrl.message}}</p>
  `,
  bindings: { 
    registerParent: '&'
  },
  controller: function($scope){
    var ctrl = this;
    
    ctrl.registerParent({childRef: ctrl});
    
    ctrl.childFunction = function(message){
      ctrl.message = 'Child function: ' + message;
    };
  }
});



app.component('parent', {
  template: `
    <h1>Parent</h1>
    <button ng-click="$ctrl.childRef.childFunction(\'called from parent\')">Notify child</button>
    <child register-parent="$ctrl.register(childRef)"></child>
  `,
  controller: function(){
    var ctrl = this;
    ctrl.childRef;
    
    ctrl.register = function(child){
      ctrl.childRef = child;
    };
  }
});

<!DOCTYPE html>
<html ng-app="plunker">

  <head>
    <meta charset="utf-8" />
    <title>AngularJS Plunker</title>
    <script>document.write('<base href="' + document.location + '" />');</script>
    <link rel="stylesheet" href="style.css" />
    <script data-require="angular.js@1.5.x" src="https://cdnjs.cloudflare.com/ajax/libs/angular.js/1.5.11/angular.min.js" data-semver="1.5.11"></script>
    <script src="app.js"></script>
  </head>

  <body>
    <parent></parent>
    
  </body>

</html>

/* Put your css in here */