Most infants and toddlers make
sporadic (and, increasingly,
predictable) demands throughout the day, and yes, they do so by screaming. I'd respectfully suggest that it's more about the parent's ability to metabolize those demands and the discipline to get back into flow than anything about handling those interrupts being a "9-5 job". For the first several months, infants aren't even especially interactive; by 6 months, they're straightforward to work around.
I am stipulating a two-parent household (but stipulating both parents work). If your focus is especially fragile (and I've worked with and admire many people form whom that's the case), a sitter makes this even more tenable. And, again, we're talking about WFH; we're not even addressing commute issues.