public void setParentOperators(
      List<Operator<? extends OperatorDesc>> parentOperators) {
    if (parentOperators == null) {
      parentOperators = new ArrayList<Operator<? extends OperatorDesc>>();
    }
    this.parentOperators = parentOperators;
  }
Implements the getChildren function for the Node Interface.
ql/src/java/org/apache/hadoop/hive/ql/exec/Operator.java