This comes from my experience of having A-level equiv. in Mandarin - I am now in my 30s.
It depends on what you seek from education, really. An A level in Mandarin will not give your child the capacity to even read a newspaper (it takes years of hard graft to learn Mandarin properly) - while the same qualification in French or Spanish would allow you to do that. I chose "reading a paper" as an example because if you can learn to do that it's a lot easier to keep a language up afterwards.
So, on a practical basis, an A level in Mandarin is useless unless the kid continues studying it later, probably full time and preferably in China. It may impress whoever reads their CV one day, but again, it's a talking point not a skill.
However, from a wider educational point of view, studying Mandarin is a perfect way to get acquainted with a totally different culture, which may well be more dominant 20 yrs down the line - it was incredible fun to do, I never regretted doing it and it has certainly opened up my mind to a different world.
So, definitely go for Mandarin, but "hedge your bets" by taking it as an extra subject to do for interest (it may be that your DCs want to continue studying it further) or by doing it as an extra-curricular activity.