New Twiddle

router\.js

import EmberRouter from '@ember/routing/router';
import config from './config/environment';

const Router = EmberRouter.extend({
  location: 'none',
  rootURL: config.rootURL
});

Router.map(function() {
  this.route('my-route', function() {
    // this.route('index'); // This is implied
    this.route('my-child-route-1'); 
    this.route('my-child-route-2'); 
  });
});

export default Router;

styles.app\.css

.outlet {
  border: 1px solid red;
  padding: 10px;
} 

.active {
  font-weight: bold; 
}

templates.application\.hbs

<h1>Application Route</h1>

<LinkTo @route="application">Application</LinkTo> |  
<LinkTo @route="my-route">My Route</LinkTo>

<br><br>

<div class="outlet">
  {{outlet}}
</div>

templates.my-route.index\.hbs

<h1>My Route</h1>

templates.my-route.my-child-route-1\.hbs

<h1>My Child Route 1</h1>

templates.my-route.my-child-route-2\.hbs

<h1>My Child Route 2</h1>

twiddle\.json

{
  "version": "0.17.1",
  "EmberENV": {
    "FEATURES": {},
    "_TEMPLATE_ONLY_GLIMMER_COMPONENTS": false,
    "_APPLICATION_TEMPLATE_WRAPPER": true,
    "_JQUERY_INTEGRATION": true
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.js",
    "ember": "3.18.1",
    "ember-template-compiler": "3.18.1",
    "ember-testing": "3.18.1"
  },
  "addons": {
    "@glimmer/component": "1.0.0"
  }
}