New Twiddle

components.my-child.js

import Ember from 'ember';

export default Ember.Component.extend({
  actions: {
    selectionChanged(value) {
      // Returns a function (it should!)
      let action = this.get('attrs.onSelectionChanged');
      
      if(action) {
        action(value);
      }
    }
  },
  items: [
    { value: 123, label: "One Two Three" },
    { value: 456, label: "Four Five Six" },
    { value: 789, label: "Seven Eight Nine" }
  ]
});

components.my-parent.js

import Ember from 'ember';

export default Ember.Component.extend({
  buttonText: "Nothing!"
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle'
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
{{my-parent}}
<br>
<br>

templates.components.my-child.hbs

<select onchange={{action "selectionChanged" value="target.value"}}>
{{#each items as |item|}}
  <option value={{item.value}}>{{item.label}}</option>
{{/each}}
</select>

templates.components.my-parent.hbs

<p>{{buttonText}}</p>
{{my-child onSelectionChanged=(action (mut buttonText))}}

twiddle.json

{
  "version": "0.9.2",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.6.0",
    "ember-data": "2.6.0",
    "ember-template-compiler": "2.6.0"
  }
}