@Override
        public String toString() {
            final StringBuilder sb = new StringBuilder();
            printNode(sb);
            return sb.toString();
        }
Need to supply a list of children as there is no point in creating an
operator node without children

@param childNodes
stroom-statistics/stroom-statistics-impl-sql/src/main/java/stroom/statistics/impl/sql/search/FilterTermsTree.java