Permission Group of the object is different from the Parent Object.
The permission group of equipment children should be same as that of the parent equipment.
Copying the children of design equipment to another design equipment in a different permission group. Then, the children in the design equipment do not have the same permission group as that of its parent design equipment.
This problem prevents the deletion of the equipment because you do not have permission to delete the child.
What You Should Do
Run the Clean Database custom command, which sets the Equipment permission group to the children.