fiddlehodl@nostrplebs.com on Nostr: How about something like "I know that in the past I've been able to work [crazy] ...

How about something like "I know that in the past I've been able to work [crazy] hours even though it was not in the job description, but starting next week I will no longer be able to do that. I can work 8 hours within the window of X to Y. Please let me know the hours that will be most useful within that window and I will make sure to be fully available."
Then you could go on to any solutions you have to the challenges that he's facing (is there anything that would help avoid the urgent problems that keep cropping up? a re-structuring of that system that you could do within reasonable hours?)

Personally I wouldn't mention the twins; the issue here is that you're being expected do to something that was not in the original job description.