function getOrCreateSubFolder(childFolderName, parentFolderName) {
  var parentFolder, parentFolders;
  var childFolder, childFolders;
  // Gets FolderIterator for parentFolder
  parentFolders = DriveApp.getFoldersByName(parentFolderName);
  /* Checks if FolderIterator has Folders with given name
  Assuming there's only a parentFolder with given name... */ 
  while (parentFolders.hasNext()) {
    parentFolder = parentFolders.next();
  }
  // If parentFolder is not defined it sets it to root folder
  if (!parentFolder) { parentFolder = DriveApp.getRootFolder(); }
  // Gets FolderIterator for childFolder
  childFolders = parentFolder.getFoldersByName(childFolderName);
  /* Checks if FolderIterator has Folders with given name
  Assuming there's only a childFolder with given name... */ 
  while (childFolders.hasNext()) {
    childFolder = childFolders.next();
  }
  // If childFolder is not defined it creates it inside the parentFolder
  if (!childFolder) { parentFolder.createFolder(childFolderName); }
  return childFolder;
}

function test() {
  // Call it looking up in Drive root folder
  var a = getOrCreateSubFolder('CHILD');
  // Call it looking up in given parent folder
  var b = getOrCreateSubFolder('CHILD', 'PARENT');
}