Hey @tryingsomethingnew lots of good advice here as usual. Just to add my thoughts (some of which reinforce earlier comments):
There will be compulsories – which vary from school to school and sometimes from year to year (DD didn't have to take MFL but DS2 did). Some schools insist on one from history/geog and MFL; others have dropped the MFL requirement. So worth knowing what happens at your DCs' school.
Triple science – yy how is it worked? A lot of schools select the most able for triple and they do it in the same lesson time – so those students get an extra GCSE. My DCs' school did this and it worked OK – even unsciency DD got 3 x A. Other schools offer triple as an option which has the advantage of more time to cover the content but then it eats up an option which will limit other choices (this would have been a big issue for DD, for example). So which does your school do? or does it offer triple as a twilight extra GCSE?
How many choices do DC get? A school local to me offers basically one free choice; I know of others where DC can choose three. DD had three choices, DS2 had two. Again this is related to compulsories and also numbers of GCSEs taken – usually 8, 9 or 10. The first school I mention offers 8; DS2 took 10 (inc triple, as above).
Will they get their choice? If not what happens? DS2 I recall had to submit four choices for two slots. I think most DC got their first two choices.
Agree no need to have A levels in mind, except that it is not the easiest thing to take A level history or geography without the GCSE (have known it tho) and yy MFL A level requires the GCSE, as do things like maths and science (but they will be compulsory at GCSE).
Could ask what is a likely grade in the teacher’s subject?
Could also just ask whether the teacher thinks they should take that subject – DS2 considered food tech but his KS3 teacher was markedly unenthusiastic about him (so he did CS instead).
I also think asking which teacher teaches GCSE and how big the GCSE group will be is useful but bear in mind that they may not know. Teachers come and go and groups change; I have had a GCSE group (MFL) of 15 and also one of 25.
Worth asking if groups are set at all; we used to set for GCSE MFL if enough took it for more than one group, with a smaller group for lower ability. But obvs if there is only one group for a less popular subject then setting is not possible.
Edited to add: meant to say hardly any GCSE subjects include coursework. If that is a strength of DC's then it’s worth looking at the ones that do – art, DT, food tech, drama, textiles, music – if appropriate.
Phew long post sorry. Hope some of it is useful.