title: How can I get all child nodes of general tree in Python
body: <p>My tree has the following structure:
  tree={'0':('1','2','3'), '1':('4'), '2':('5','6'), '3':(), '4':('7','8'), '8':('9','10','11')}</p>
  
  <p>How can I wrote Python code to retrieve all given child nodes of a particular node? 
  For example, if I give it node 4, the code should retrieve 7,8,9,10,11. 
  For node 2, it should retrieve 5, 6 and so on.</p>
  
  <p>I just started learning the basics of Python but I have no idea how to implement this for non-binary trees..</p>