Where I live in Berkshire we are on the border with Buckinghamshire, and both mine attended a Bucks grammar, which was right for them. Many of my friends choose not to put their offspring through the test, and send theirs to a nearby town where there are comprehensives, as the local non-selective has a poor reputation. Doing well at a comprehensive is definitely preferable to struggling at a grammar, but if the area doesn't have comprehensives nearby, a lot will depend on the standard of the local non-selective. The non-selective I work in gets results as good as many comprehensives, despite some of the brightest pupils being 'creamed off' to the grammars.
Unfortunately there is no avoiding exams for ever. Our education system is so focused on GCSEs these days that all kids face huge pressures in secondary school. I work as a learning support assistant in a good non-selective but some of our pupils will never pass a GCSE at the equivalent of A-C despite all the support we give them, yet they still have to sit them. I personally think that is extremely destructive of their confidence as they simply learn to fail! My school do offer separate invigilation for some students who find being in the main hall for exams particularly stressful, but some schools may not be able to do that due to staffing or room availability. When the time comes it may be worth discussing that with whatever secondary school she attends. Whether or not she needs to take this exam at this stage will depend on many factors, particularly what the school options are in the area and also, very importantly, where her friends will go, as that may be particularly important to her. I think the most important thing you can say to her is that whether she passes or not, as long as she has tried her best that is all that matters, and you will be proud of her and love her just as much whatever happens.