I'm surprised about the assumption of less efficiency from a wahm. Having four children myself, I have gained an enormous set of skills specifically geared toward greater efficiency. What used to take me two days to write, now takes me an afternoon; what used to take me many days to draw/paint/sculpt, now takes me a day or less. I have been able to plan for less time to production than ever before having children.
However, the overall amount of time I have for such things is reduced. BUT as far as efficiencty is concerned, I am a well-oiled machine!

I have found that the biggest reason for this is that I ruminate over ideas and plans while I am doing other things so that when I have a spot of time to work, I am already in the finishing phase of a project and can accomplish it straight away; there's no lag time at all.
I think if you are employed and paid by the hour, this could be a problem, but if you are paid according to accomplishment, then what you do with the time you are not working directly on your job is really none of anyone's business.
I have found that hourly work is a huge rip-off for me because I am so efficient; why should I be paid less because I can accomplish so much in so little time? I always opt for contract work at flat rates for projects rather than hourly wages. If my estimates are wrong, and it takes me longer than expected (not past a deadline of course, but just more hours of my time), then I suck it up. I didn't do that more than once anyway because I learned how to make sure I am estimating accurately and to err on the side of my benefit so that if I had any surplus, I could use that to polish things up even more and offer other benefits to my clients.
On the topic of working with a newborn, I couldn't do it; mine are very closely spaced, and my first non-high-needs newbie was ds3, followed by another high-needs babe with ds4. I have just recently returned to working (at home and in my studio) in this past year after seven years hiatus, although I did a few contracts within those seven years (at-home, short-term single assignments).
I thought I would keep working from home after ds1 was born, but had to stop when I was in my second trimester for exhaustion and then only began to recover last year (6 yrs later). That was shocking and not at all what I had envisioned. Also, I didn't account for the chaotic life-management of my dp, including his utterly chaotic career; that put the brakes on for sure, as well. There are so many things that I didn't know would happen, and couldn't have accounted for before, that I have now through experiencing them.