My child has cancer, can there be complications with fifth disease?

Possibly. If your child is on an active treatment protocol that affects the immune system, then the development of any infection can be serious. Your child might get more sick from the disease than someone else who has a normal immune system and is not battling cancer.
Maybe. Kids get a rash, mild fever, and/or mild cold symptoms. Adults get a rash, some joint aches, and/or joint swelling. Infected persons, with or without symptoms, mostly recover without complications and have immunity against re-infection. Complications can occur in persons with immunosuppression, such as some cancer patients, and in patients with chronic anemia.