Mine all went through 11+ some years ago. One of them has always struggled with exams and did right up until the final one this year. The other 2 tend to cope better.
If they are worried then its definitely better to talk it through as much as they need but also try to keep grounded in reality. Sadly it is one of the few exams where they don't really get a second shot ( except Cambridge finals
but its still important to make it clear that 1. You won't think less of them if they don't get a place at GS and 2. In the wider scheme of things it isn't the end of the world ( there are safe and well and with you- not in Syria or Iraq....)
It does matter which school they go to but you will deal with the outcome together (- whether its trying again for places that come up or finding ways to make the most of where they do go, scholarships/bursaries etc.) But I wouldn't dwell on the detail at this point
Tell them that this year is a new exam so nobody really knows what to expect so if they find it hard so will everyone else. The important thing is to stay calm and do their best. Picking up as many marks as they can and not worrying about what they can't do. I know lots of friends over the years whose children felt they had awful exams ( and a few did do much worse than usual for them) but they still did well enough to get s GS place so don't at any point give up in the exam.
We didnt do anything for a few days before the exam and had a cosey family evening on the Friday with a treat for afterwards ( nothing too demanding as they were all shattered!) and then didn't discuss the exam afterwards.
Its sad they have to go through this but it will soon be over.