RosieIrene, can you contemplate a career outside the magic circle? I am a partner in the London office of a national firm, and got my equity 8 months PG with DC2. Even if you knew which firm I worked for, this alone would not be enough to identify me - how much of a contrast is that with your place? It can be tough here sometimes, but it's not as bad as the MC. I think mid-tier firms have to be more flexible to attract and retain good lawyers. This is gradually feeding through to my colleagues, and this year we promoted two part-timers to partnership (I am FT), and we have recruited a PT partner, and offered to another candidate who was PT.
I agree with Anchovy, the more senior you are the easier it is, because you get more control over your work. Whether you will ever get enough control is something determined by your clients, IME. One of my most demanding clients has a corporate culture of not working after 5.30pm or at weekends - I still need to draft docs in the evening for them sometimes, but there's no late night completions.
The thing that really will be career suicide is for you is the fact that it is always you taking time off or coming in exhausted when your DCs are ill/ going to the dentist/ whatever. Probably not what you want to hear. You really do need to work out what a successful life is for you, and your family, and I hope once you have done that things will be clearer.
Another point I would echo is that things will change as your DCs get older. I have always worked FT, but now DS1 is starting school, I find I want to know more about that part of his life, so I'm working from home one day a week (no-one has noticed yet ) and might go to 4 days sometime next year. Maybe this will help you, if you think it's not forever.