New Twiddle

components.checkbox-treeview.js

import Ember from 'ember';

export default Ember.Component.extend({
  items:[
    {id:1, label: "project1", isExpanded: false, children: [
     {label: "unit1"},
     {label: "unit2"},
     {label: "unit3"}
    ]},
    {id:2, label: "project2", isExpanded: false, children: []},
    {id:3, label: "project3", isExpanded: false, children: [
     {label: "unit1"}
    ]},
  ],
  actions: {
    expand(id) {
      this.items.forEach(item => {
        if(item.id === id) {
          Ember.set(item, "isExpanded", !item.isExpanded);
        }
      });
    },
    check() {
    
    }
  }
});

components.header-treeview.js

import Ember from 'ember';

export default Ember.Component.extend({
  classNames:["header-treeview"]
});

controllers.application.js

import Ember from 'ember';

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

styles.app.css

body {
  margin: 12px 16px;
  font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif;
  font-size: 12pt;
}

.header-treeview{
	color: white;
  background-color: lightblue;  
}
.header-treeview>div {
	display:inline-block;
  padding: 5px;
}

.header-treeview__checkbox {
	border-right:1px solid white;
}

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{checkbox-treeview}}
<br>
<br>

templates.components.checkbox-treeview.hbs

{{#each items as |item|}}
	{{header-treeview 
    item=item 
    expand=(action "expand") 
    check=(action "check")
  }}
  {{#if item.isExpanded}}
    {{#each item.children as |child|}}
      <input type="checkbox" value="Bike"> {{child.label}}<br>
    {{/each}}
  {{/if}}
{{/each}}

templates.components.header-treeview.hbs

<div class="header-treeview__checkbox"><input type="checkbox" value="Bike"></div>
<div class="header-treeview__label" {{action expand item.id}}>{{item.label}} {{if item.isExpanded "^" "v"}}</div>

twiddle.json

{
  "version": "0.15.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.2.2",
    "ember-template-compiler": "3.2.2",
    "ember-testing": "3.2.2"
  },
  "addons": {
    "ember-data": "3.2.0"
  }
}