I have noticed a tendency to this nighttime routine together thing.
So, I do think with trades, while normally they have set hours, there is also an expectation that you don't leave people without vital things like a toilet, or water, or electricity. Not unless it's been made clear beforehand that is the arrangement, or it really can't be done.
It's easy enough to think people can just go to a hotel or something but that is a huge problem for many financially, especially for four days!
I would expect a plumber in that situation to stay on unless it was going to be a much longer job, and I'd expect myself or husband to stay on in that kind of circumstance. While I am not a plumber, I do have a job where similar kinds of things happen and I expect to have to stay.
In this particular case, if it is just hot water, I wouldn't necessarily think that was a necessity, though heating might be depending on the time of year. Though leaving the hot water off until Tuesday is a bit much.
I also wonder to what extent the situation is down to his misjudgment - would they have started the job if he had estimated correctly that it would mean the weekend with no heat and hot water? Is that a failure on his part ? If so I he has a responsibility to follow through.
I do find the idea that one parent can't bathe the kids alone silly unless there are other issues at play, which of course is possible. Although "his turn" seems to indicate that's not it. It seems a very inflexible arrangement, surely it's better if it's possible for both parents to do bedtime alone when something else comes up? Rigged arrangements with kids don't usually serve either parent in my experience.
A new business might not be so compatible with that kind of rigidity either. I'd certainly never recommend a plumber who came, started a job, and just took off like that.
My last thought is, I think if this situation was the other way around, MN would be all over him for not being able to do bedtime for two kids alone.