Your happiness, long term, is part of your children's well being. They won't be little forever, and you pursuing your own life goals will make you a better parent.
So rather than this job, focus on what you want out if life. In the long term.
As an older child and teenager I felt the pressure of a mother who was overly focused on us, for a while when she did have a more demanding job and it was quite freeing!
Think widely. This job could be good, or a stepping stone back in which enables you to change to something else quite soon:
What about this profession, different job or way of working? I've used a couple of network law firms which offer more affordable advice to clients and, I imagine, freelance flexible working for legal mums.
Or a change of direction when both your kids are at school, starting your own business in a similar or different field?
What do you like about working? Money? Challenge? Freedom? Achievement? Structure? Friendship?
I agree with sheryl Sandberg, if you're going back, get the best job you can. But 'best' for you isn't necessarily senior/ demanding / well paid (though it might be).
If you could magic your ideal job now, or in five years time, what would it be like?
Also childcare doesn't have to be second best. If you can afford it you can find and hire someone you like and can bring things to your kids. Once I got the hang of it I enjoy having a nanny, someone who is as obsessed with pfb as I am, who brings new ideas and energy.
Working works for me, now and for the long haul. Makes me better mother. Stand back and think about what would make you happy and purse that.
Also: my DH also had a demanding job (city) though I am part time I carry responsibility at work. I am v robust that he does a share (he does mornings) and that his job is not more important, so sometimes he has to push back on his work demands as I've got, say, an evening commitment. It's not easy for him but he does it, bless him. If you are going back to a job with responsibilities, even part time, your DH needs to help out at home more. For me, it doesn't have to be 50/50 but it does have to be shared, otherwise it's too much pressure.
Good luck whatever you decide