Adds a child to the specified parent.

def AFF4AddChild(self, subject, child, extra_attributes=None):
    """Adds a child to the specified parent."""
    precondition.AssertType(child, Text)

    attributes = {
        DataStore.AFF4_INDEX_DIR_TEMPLATE %
        child: [DataStore.EMPTY_DATA_PLACEHOLDER]
    }
    if extra_attributes:
      attributes.update(extra_attributes)
    self.MultiSet(subject, attributes)