I understand your dilemma, OP.
I’m a doctor (registrar) married to a doctor (consultant). So, my husband earns a lot more than me, though not nearly as much as your husband, and I suppose my earning potential is pretty similar to his in the long term. We have three children aged 6, 4 and 2. Eldest is in year one at school and younger two at nursery.
We have really struggled to get a balance that feels right between work and family life. Before the children were born, I presumed I’d press on in my career - it’s such a big part of who I am - and use whatever childcare needed to make this work. I was, probably naively, taken by surprise to find that I didn’t want to do this. If anything, it’s actually got harder as time has gone by as my eldest seems to need a parent around more now than she did as a toddler. She wouldn’t do well in full time wraparound care, though I know some children are totally fine.
My husband does as many school and nursery runs as possible and is a brilliant dad and partner, but it simply wouldn’t work financially for him to massively reduce hours. These threads always suggest it, and I understand the principle, but the whole family benefits from having one really good salary.
I have ended up having to go backwards and change my specialty as I couldn’t find a way to make my old job work in a way that suited everyone. I also find that working three days feels like enough time to maintain some semblance of a career, whilst being around as much as possible for my children. I still feel guilt about not being there more, though, and guilt about letting myself and feminism down by being the one to take a career hit.
In your position, given that you clearly love your job and it sounds like a possible specialist/CNS type role that would be hard to return to, I’d consider getting a nanny or nanny housekeeper in the first instance. This might help with the problems you describe with your youngest. I also wonder whether dropping a day or reducing the length of your working day would be possible? I’d try these options before quitting! Though, ultimately, if the children are really struggling I think I would end up stopping work and maybe doing a bit of bank alongside an additional qualification with a view to returning to a similar role in a few years.