I agree with Janeite; unless there are very serious reasons your particular dc cannot cope with exams, it's a bad precedent to let them opt out (and even worse would be to let them know you are lying to the school about illness or something). Sooner or later, when they are at work or at college, something equally stressful will come up, and then you will have taught them a most undesirable solution to stress.
I would also like to say that SATS don't have to be a crushing experience. My dd hasn't done hers yet, but she is actually enjoying the preparations because she has very enthusiastic teachers who see it as a challenge to turn it into a positive experience. I don't know whether they personally approve of SATS or not, but they are certainly making the best out of them.
For dd, it is a useful time to focus her work, to look at what areas she needs to try harder in, and to learn a bit of exam technique (e.g. to answer the question you are asked rather than the one you would like to be asked). This hopefully will make GCSEs less of a shock when they come.
It also seems to be breeding a certain amount of camaraderie in her set; they are all working towards a common goal. There seems to be very little competition and jealousy, which must be due to the attitude of the school. And friends whose dc's sat the exam last year tell me their offspring did not find it a stressful experience.
So I would say, don't assume that it will stress out your dc. Sometimes children pick up unnecessary fears from their parents.