title: count the nodes of a child in the firebase
body: <p>I have a structure like this in firebase:</p>
  
  <p><a href="https://i.stack.imgur.com/eZgSG.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/eZgSG.png" alt="enter image description here"></a></p>
  
  <p>Inside the node 'postagens-curtidas', I need to count this childrens and save this value at the 'qtdCurtidas' node, as the photo shows.</p>
  
  <p>Is there a way to make this?</p>