@Examamum so sorry that your DC did less well than they hoped. How frustrating for them and you too especially as you did so much to support.
Are parents typically coaching their children on each subject?
I mean no. For one reason, no bc they won't have the expertise, knowledge and exam intel. I taught secondary MFL and tutor that subject now so I would be fine on French and German; I have also supported numerous students in Eng lit and lang so would be OK there too, but beyond that, GCSE physics err no.
And ofc most parents are not teachers so would not even have the level of expertise that I can lay claim to.
So what can you do? I suggest that the key is, how much actual active revision did your DC do? Revision guides are great but if the YP doesn't open them os use them then they won't help.
Tutors are helpful too – what kind of feedback did you receive? What sort of PGs did they suggest? Were they teachers, did they have up to date exam spec knowledge?
The way to do well at GCSE is to understand fully what the examiner is looking for. So to take my subject, the conversation in German will not be natural – you will get extra marks if you expand and develop an answer in a way that seems odd – but it's how you do well. There are certain words and terms to use, boxes to tick, in all subjects. That's what the YP needs to know.
Good teachers should be telling them this. They need to revise actively (make flash cards and get you to test them, summarise a topic) and in a targeted way ("The past tense in French; causes of WW1" rather than just "revise history") and then do practice papers to see where the gaps are - then work on those gaps. HTH