use .items instead of .children.

Prevents gotchas when using `<template repeat>` inside of <core-pages>. Template will be counted as a child.

demo.html

@@ -111,15 +111,15 @@
 
   <script>
     document.querySelector('#first').onclick = function(e) {
-      this.selected = (this.selected + 1) % this.children.length;      
+      this.selected = (this.selected + 1) % this.items.length;      
     };
 
     document.querySelector('core-pages.fancy').onclick = function(e) {
-      this.selected = (this.selected + 1) % this.children.length;
+      this.selected = (this.selected + 1) % this.items.length;
       this.async(function() {
         if (this.selectedIndex == 0) {
           this.selectedItem.classList.remove('begin');
-        } else if (this.selectedIndex == this.children.length - 1) {
+        } else if (this.selectedIndex == this.items.length - 1) {
           this.items[0].classList.add('begin');
         }
       });