Hi OP,
Everyone situation is different and I'm going to give you pros of cons of quitting your job to care for your children full time.
Personally, I do not think you should quit. I would highly consider doing part time (which your already doing now) and moving onto another speciality which is much less demanding. I would strongly consider this. I think you are very fortunate to be in a field where you can specialise in other areas. Please use this to your advantage.
I totally understand your point about not being there for your children, stressed out at home life. We have all been there. But I would not quit-no. I love my DC so much (I work part time) but I have my own needs, and my career is part of my identity- I'm not just a "mum". Also, children will need you throughout their years, it doesn't stop when they are teens. Do you not hear that a higher rate of teenagers are suffering form mental illness? Also considering exams, relationships, new found independence. I believe that teens, may not need you physically, but they require a LOT of emotional support.
My mum worked when I was younger and honest to God, I cannot even remember. However, I did find it a bit lonely at times where she would come back from work late (8pm). When I went to primary school, my mum didn't work. I liked having her here. But looking back...I wish she did something for herself. We had no money to do extra curricular activities, I know many people say this is not important yadada... but a young girl who was bullied, shy and didn't have much friends (just her annoying siblings for entertainment) I do believe this would have boost my confidence.
Also, our mother still donates our lives. We are all adults but she has this yearning need to want to control aspects of all our lives. I do believe that this has resulted from being a SAHM.
Please OP do not give up. Consider hiring a tutor for your son if he has reading needs. My son has delays in his reading, when I come back home in the evening, I we do some reading stuff together. You can even instruct your nanny to help you with this for your son.