Do both parents have to have sickle cell trait in order for their child to have sickle disease?

99+% yes: The answer is yes, but there is always a chance of a new mutation (rare). In which case one of the parents might not have trait. There is also hemoglobin c which can combine with hemoglobin s to give a very similar clinical disease (hemoglobin sc disease). You can also have sickle beta thalassemia which is very similar to sickle cell disease. But only one parent has to have sickle trait.