Child component constructor called twice

import { NgModule }      from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';

import { ParentItemComponent } from './app.component'
import { ChildItemsComponent } from './children.component'

@NgModule({
  imports:      [ BrowserModule ],
  declarations: [ ParentItemComponent, ChildItemsComponent ],
  bootstrap:    [ ParentItemComponent, ChildItemsComponent ]
})

export class AppModule { }


/*
Copyright 2016 Google Inc. All Rights Reserved.
Use of this source code is governed by an MIT-style license that
can be found in the LICENSE file at http://angular.io/license
*/

import { platformBrowserDynamic } from '@angular/platform-browser-dynamic';

import { AppModule } from './app.module';

const platform = platformBrowserDynamic();
platform.bootstrapModule(AppModule);


/*
Copyright 2016 Google Inc. All Rights Reserved.
Use of this source code is governed by an MIT-style license that
can be found in the LICENSE file at http://angular.io/license
*/

/* Master Styles */
h1 {
  color: #369;
  font-family: Arial, Helvetica, sans-serif;
  font-size: 250%;
}
h2, h3 {
  color: #444;
  font-family: Arial, Helvetica, sans-serif;
  font-weight: lighter;
}
body {
  margin: 2em;
}
body, input[text], button {
  color: #888;
  font-family: Cambria, Georgia;
}
a {
  cursor: pointer;
  cursor: hand;
}
button {
  font-family: Arial;
  background-color: #eee;
  border: none;
  padding: 5px 10px;
  border-radius: 4px;
  cursor: pointer;
  cursor: hand;
}
button:hover {
  background-color: #cfd8dc;
}
button:disabled {
  background-color: #eee;
  color: #aaa;
  cursor: auto;
}

/* Navigation link styles */
nav a {
  padding: 5px 10px;
  text-decoration: none;
  margin-top: 10px;
  display: inline-block;
  background-color: #eee;
  border-radius: 4px;
}
nav a:visited, a:link {
  color: #607D8B;
}
nav a:hover {
  color: #039be5;
  background-color: #CFD8DC;
}
nav a.active {
  color: #039be5;
}

/* items class */
.items {
  margin: 0 0 2em 0;
  list-style-type: none;
  padding: 0;
  width: 24em;
}
.items li {
  cursor: pointer;
  position: relative;
  left: 0;
  background-color: #EEE;
  margin: .5em;
  padding: .3em 0;
  height: 1.6em;
  border-radius: 4px;
}
.items li:hover {
  color: #607D8B;
  background-color: #DDD;
  left: .1em;
}
.items li.selected:hover {
  background-color: #BBD8DC;
  color: white;
}
.items .text {
  position: relative;
  top: -3px;
}
.items {
  margin: 0 0 2em 0;
  list-style-type: none;
  padding: 0;
  width: 24em;
}
.items li {
  cursor: pointer;
  position: relative;
  left: 0;
  background-color: #EEE;
  margin: .5em;
  padding: .3em 0;
  height: 1.6em;
  border-radius: 4px;
}
.items li:hover {
  color: #607D8B;
  background-color: #DDD;
  left: .1em;
}
.items li.selected {
  background-color: #CFD8DC;
  color: white;
}

.items li.selected:hover {
  background-color: #BBD8DC;
}
.items .text {
  position: relative;
  top: -3px;
}
.items .badge {
  display: inline-block;
  font-size: small;
  color: white;
  padding: 0.8em 0.7em 0 0.7em;
  background-color: #607D8B;
  line-height: 1em;
  position: relative;
  left: -1px;
  top: -4px;
  height: 1.8em;
  margin-right: .8em;
  border-radius: 4px 0 0 4px;
}
/* everywhere else */
* {
  font-family: Arial, Helvetica, sans-serif;
}


/*
Copyright 2016 Google Inc. All Rights Reserved.
Use of this source code is governed by an MIT-style license that
can be found in the LICENSE file at http://angular.io/license
*/

<!DOCTYPE html>
<html>
  <head>
    <title>Angular QuickStart</title>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <link rel="stylesheet" href="styles.css">

    <!-- 1. Load libraries -->
     <!-- Polyfill for older browsers -->
    <script src="https://unpkg.com/core-js/client/shim.min.js"></script>
    <script src="https://unpkg.com/zone.js@0.6.25?main=browser"></script>
    <script src="https://unpkg.com/reflect-metadata@0.1.8"></script>
    <script src="https://unpkg.com/systemjs@0.19.39/dist/system.src.js"></script>
    <!-- 2. Configure SystemJS -->
    <script src="https://cdn.rawgit.com/angular/angular.io/f2daab7/public/docs/_examples/_boilerplate/systemjs.config.web.js"></script>
    
  </head>

  <!-- 3. Display the application -->
  <body>
    <parent-item parentItemId="1">Loading</parent-item>
  </body>
</html>


<!-- 
Copyright 2016 Google Inc. All Rights Reserved.
Use of this source code is governed by an MIT-style license that
can be found in the LICENSE file at http://angular.io/license
-->

import { Component, ElementRef } from '@angular/core';

@Component({
    selector: 'parent-item',
    template: '<child-items [parentItemId]="parentItemId">Loading...</child-items>'
})
export class ParentItemComponent {
    public parentItemId;

    constructor(elm: ElementRef) {
        this.parentItemId = elm.nativeElement.getAttribute('parentItemId'); 
    }
}

import { Component, Input } from '@angular/core';

@Component({
    selector: 'child-items',
    template: "child display"
})
export class ChildItemsComponent {
    @Input() public parentItemId: number;

    constructor() { }

    ngOnInit() {
        if (this.parentItemId) {
            console.log('Parent Id: ' + this.parentItemId);
        }  
        else {
            console.log("Parent Id not set!");
        }
    }
}