During the schooling of my children, my opinion was that only the students who are not well prepared for an exam have jitters. Though, I did not rule out a small dose of excitement, which, in my opinion, could be stimulating.
Later, when I started teaching, I realized that well-learned material is important to reduce the fear of exams, but for some children that is not enough and that there are many factors that influence the over-anxiousness or lack of self-confidence. Children often have a fear of betraying the expectations of their parents, which are sometimes unrealistic.

Do your children have a jitters?
How does it manifest?
How do they cope with the jitters?