public void sortChildrenById() {
        if (sortType == SortType.BYID) {
            return;
        }
        children = sort(children, Group::compareId);
        sortType = SortType.BYID;
    }
Sorts the children by their id, if they are not sorted already.
searchlib/src/main/java/com/yahoo/searchlib/aggregation/Group.java