Not sure how to "draw a clear and inviolate boundary" - "Sorry, but you're responsible for the kids during the day so suck it up and write at night"??
It depends on how you communicate, right? A generic version:
I think it's great that you've got this book project going on. It's just like my [fill in the blank]. It's helping you establish your new identity in the midst of our separation and divorce. That's terrific. And I sure don't envy you the challenge of finding time to write; I've seen here and there on the internet a lot of discussions by writers of confronting that challenge. But I have every confidence that you'll manage it. Just like I'm managing to find the time to [do my GAL stuff]. It won't be easy -- just like it's not easy for me to [GAL], but I wouldn't give those things up now, since they're so meaningful and important to me. But I'm totally confident that you won't give up on this writing, just because you have to X, Y, and Z during the day.
Hey! I just thought of something! Would you be interested in the links for some of those things I saw on the Web? Because I'd be happy to -- oh, look at the time! I've got to go to [spinning, basket-weaving, work, the gym, drive over to the 7-11 and have a Slurpee].
