It's not all the hype everyone gives it
Students often hear that the Leaving Cert is "the hardest thing you'll ever do" and "you'll remember it for the rest of your life". It's important to remember this is not the case; the Leaving Cert is just a stepping stone towards the next chapter. Your child is young and has their whole life ahead of them. If they are stressing about the exams, they need only remember that after all of it, when they are graduating college or working in their first job, they won't even be thinking about the Leaving Cert.
As for remembering it for the rest of their lives, it's not likely. My advice is this: once an exam is done, don't think about it any more. It's over so there is no point in stressing over something they have no power over. I know it’s tempting, but dissecting the exams with friends or family afterwards usually makes people feel they did worse than they actually did.