So in what case do parents produce sickle cell children?

Both parents: Both parents must carry sickle cell trait to have a child with sickle cell disease.

See below: Sickle cell disease is ss. Sickle cell trait is as. Each parent must contribute an s to the child if that child will have ss. That means that each parent must either have as (trait) or ss (disease). The odds of having ss vary depending on what each parent has.