jsbin.hiquh.html

 
<polymer-element name="x-foo">
  <template><content></content></template>
  <script>
    Polymer('x-foo',{
      attached: function(){
        this.async(function() {
          debugger;
          console.log(this.childElementCount + "/" +   this.shadowRoot.querySelector('content').getDistributedNodes().length );
        });
      }
    });
  </script>
</polymer-element>