Right.
Firstly, teachers don't necessarily just teach their degree subject. My main teaching subject (English) - is not my degree subject (Classics/Latin).
I've also taught Drama, Geography, History & PSHCE. I was crap at Drama (takes a special sort of nutter dedicated person, does that! ) - but in none of the others has my knowledge of the subject ever been lacking.
Also, if you get a 3rd it doesn't mean you 'know less than half of the subject'! It means you got less than 50% in your finals, I believe, which isn't quite the same thing. The 'subject' is, hopefully, a bit broader than the course...('OK, I know 50% of Physics - Hawking, eat your heart out! ).
I am quite open about my richard. Early on in my degree course, 20 years ago now, I went off the rails & spent most of my final year managing a local music venue.
After Uni, I worked in the licensed trade for 10 years before turning my hand to teaching; I did an absolutely storming interview in order to be accepted onto an English PGCE.
My subject knowledge has always been mentioned as a key strength whenever I've been observed/performance managed/Ofsteded.
Thirdly, teachers are, or should be, life long learners. Degree knowledge has a sell by date in many subjects.
& finally: good classroom practitioners are people who have ENOUGH subject knowledge, but can also engage a class, control any poor behaviour, monitor progress & set targets.
With all due respect to subject specialists, I've known an English teacher on long-term supply deliver KS3 Science with notably more success than the designated teacher who was academically brilliant, but simply couldn't cope with the kids, the paperwork, the hours.
I do know one teacher who probably couldn't deliver an A* in our teaching subject if you plonked her in front of the GCSE paper, much less teach one. However, luckily for your kids she's now management & does very little teaching!
However, as for the rest of us,
I'm 'performance managed' on whether the children I teach reach or exceed their targets. If children who were expected to excel didn't, I'd've long since been forced out of my job.
It's probably fairly moot now, as I can't see teacher training courses being swamped for a few years - there aren't going to be the jobs.
Honestly. Of all the things to worry about re: your children's teachers, the class of their degrees needs to be waaaaay down the list!