Vue.component( 'parent-component', {
  template: '#parent-component',
  data: function() {
    return {
      testObject: {
        val: 'Test Value'
      }
    }
  },

  methods: {
    // Added this method to listen for input event changes
    onChange(newValue) {
      this.testObject.val = newValue;

      // Or if you favor immutability
      // this.testObject = {
      //   ...this.testObject,
      //   val: newValue
      // };
    }
  }
});

<script type="text/x-template" id="parent-component">
  <div>
    <child-component :test_object="testObject"
      @inputChange="onChange"></child-component>

    <p>This is in the parent component</p>
    <p><code>testObject.val = {{testObject.val}}</code></p>

  </div>
</script>

<!-- Instead of v-model, you can use :value and @input binding. -->
<script type="text/x-template" id="child-component">
  <div>
    <label for="html_input">HTML Input</label>
    <input type="text" name="html_input"
      :value="test_object.val"
      @input="$emit('inputChange', $event.target.value)" />
  </div>
</script>