title: How to make a dynamic room for multiplayer quiz app using firebase?
body: <p>I am making an app.. I mean a game type app. it's a quiz app where two people can
  pair them up and can answer questions one by one and so on.
  what I want is:
  I want to make a virtual room,
  when USER1 want to play he will click search competitor, a virtual room with random string key will be made with one player.
  then when USER2 will click on find competitor button his room will also be created and he will also search all rooms created before him, if any room have less than 2 users then he will join that room and start playing.
  I am new so I am not sure if that is good approach but that's all I have planned.
  how to count childs of a node (node that we does not know whats its key..
  )
  see attached image</p>
  <p><a href="https://i.stack.imgur.com/1vuVv.png" rel="nofollow noreferrer">please find image here</a></p>