What does I need to tell my child when older about having had primary dysmenorrhea?

See a OB-GYN. Tell your daughter that severe cramping is not normal and often has a cause such as endometriosis or other pelvic pathology. If that occurs, please make sure she is seen by an ob-gyn for an exam, especially if cramping is not relieved by prostaglandin otc inhibitors such a Ibuprofen or anaprox (naproxen).
No need to suffer. If like you, your child ends up with primary dysmenorrhea (pd), the most important thing to advise them is that it is a treatable disorder. A good gynecologist has many options in both the evaluation and the treatment of this problem. There is no need to suffer, miss work or school. One should not only function when dealing with pd, but one should actually carry on and have an enjoyable day.