I do finance in a secondary market. I'm also a single mom with a special needs kid who is old enough to notice when mom is and is not around. I'm not sure that averaging 55 hours per week is a realistic goal, and I doubt that you'll be as efficient as you think you will be; if that's a firm line, depending on what type of hour requirement you have, it may not be a great fit for your lifestyle.
My partner is big on face time, so I try to be here when he is. Usually come in later in the morning between 9:30 and 10:30 since he gets in late. This buys me a little more time with my kid in the morning, if only so that we can eat breakfast together. I am usually here until about 7 or 8. I don't live that close to my office, so by the time I get home, I have at most 2 hours with him. Most of that time it spent with me sitting on the couch because I am exhausted instead of actually interacting with him as I would want to.
The best things that I have figured out are to try to match my partner's schedule so that I'm not physically in my office more than I need to be. If my partner leaves early, I try to leave early. If I know he's going to be out for the day, I try to keep my day short.
For turning down work gracefully -- someone already answered this one. The best thing would be having a partner who can step in and control your workflow. If you don't have that, be upfront with people: tell them you'd be happy to do it, but you have x, y and z, so if they are on a tight deadline, you may not be able to deliver a timely, quality work product. Usually, they will go to someone else if they need it urgently or they are fine with giving you a couple of extra days.
FWIW, if spending time with your kids is something you value, all of these tips won't be helpful. The difficulty of the work and the hours pale in comparison to the overwhelming sense of guilt that I have about not being there every day for my kid. I worry whether my kid will resent me for not being there for him. I don't like that, on top of before and after school care, I still need a nanny to pick him up every day and do all of the things with him that I should be doing. I feel an enormous amount of weight over whether I am making the best decision for the both of us by doing this. I felt awful when my kid told me that, when he grows up, he doesn't want to be a lawyer because he sees how much I work and how little I have time for other things.