New job internal to company - how to propose part time?

I loathe my current position but was recently approached about a new role in the same company (biotech) that would be much more rewarding. If I were to push forward, I'm sure I could get a raise (they are really set on having me in the job). The only trouble is, I am 5 months pregnant and am really starting to get sad about not having all of my attention to devote to DD. The idea of working 3 dpw sounds perfect, as I could still keep my brain active and bring in some cash (and keep some semblance of a maternity leave), but get two days a week to spend with DD to take her to classes, the library, etc. As it is now, I get home and it's basically time to do dinner and bed.

So, I've (pretty much) decided that I don't want to take the position if it would be full time. However, I'd really like to take it if I can get it at 3 dpw. Any idea on how to approach the hiring manager with this proposal? I've already scoped out which portion of the job I could take, and as far as I understand, there are existing resources to cover the rest. She is also a mom of 2 young kids but went back to work after 6 weeks with her second and is an executive in title and personality, so not sure if she'll consider it or kick me out the door... If it is the latter, I either have to stay in my current position (ugh) or quit entirely and hope to dig up some consulting work. That would mean losing our nanny and being pretty tight financially, but would certainly have more time with DD! Enough rambling - any thoughts on a good way to propose this?