Delete 1:1 parent form child entity without JPA mappings

I have a situation where I need to remove any associated parent when a child entity is removed and vice versa. But for “performance” reasons, the owner has not included any kind of JPA mappings in the entities. If i manually remove the parent first before deleting the child, it fails due to FK constraint and vice versa. Can anyone help?
Entities:

@Table(name = "child_node")
public class ChildNodeEntity {
    @Id
    private Long id;

    @Column(name = "case_id")
    private String caseId;

    @Column(name = "parent_id", updatable = false)
    private String parentId;
}

@Table(name = "parent_node")
public class ParentNodeEntity {
    @Id
    private Long id;

    @Column(name = "case_id")
    private String caseId;
}

Here in the child entity, parentId is kept as String (to avoid performance issues plus having the parent object has no use anywhere in the project).

Here’s the code for deletion of parent from child:

@Transactional
private void deleteBauCase(String caseId) {
    ChildNodeEntity child = service.getChildNodeEntityById(caseId);

    ParentNodeEntity parent = service.getParentNodeEntityById(caseId);
    parentNodeRepository.delete(parent);

    childNodeRepository.delete(child);
}

What should be done to delete both these entities? And the reverse case as well (delete child from parent).
I cannot use @OnetoOne mappings and cascade delete. Is there another way to execute this? The foreign key constraint is there on the database side. Please help as im only familiar in working with annotations.