public class ParentService {

    public getParentList(String parentSetLookup) {
        List<Parent> parentList = getParentList(parentSetLookup);
        List<Integer> parentIdList = getParentIds(parentList);
        List<ChildOne> childOneList = getChildOneList(parentIdList);
        List<ChildTwo> childTwoList = getChildTwoList(parentIdList);
        List<ChildThree> childthreeList = getChildThreeList(parentIdList);

        mapChildrenToParent(parentList, childOneList);
        mapChildrenToParent(parentList, childTwoList);
        mapChildrenToParent(parentList, childThreeList);

        return parentList;
    }

    private void mapChildrenToParent(List<Parent> parentList,   List<ChildBaseClass> childList) {
        for (Parent parent : parentList) {
            for (ChildBaseClass child : childList) {
                if (child.getParentId().equals(parent.getParentId())) {
                    %%%parent.getChildList().add(child);%%%
                }
            }
        }
    }

public class GrandParent {
    List<Parent> parent = new ArrayList<>();

     // Getter and Setter
}

public class Parent {
    private Integer parentId;
    private List<ChildOne> childOneList = new ArrayList<>();
    private List<ChildTwo> childTwoList = new ArrayList<>();
    private List<ChildThree> childThreeList = new ArrayList<>();

    // Getters and Setters
}

public abstract class ChildBaseClass {
    private Integer parentId;

    // Getter Setter
}