Zero breaks, for one. And no ability to take sick days/stop doing it if you're injured. Doing work for 1-2 hours a day or a few hours a week is very different from doing it for 20-40 hours a week. (Much like there's a difference between taking on a project or two for a small team that requires some programming and being a full time dev). Oh, and you can't sit. That's like saying anybody who mows their lawn would have no difficulty in a landscaping position.

And the kid thing it strongly depends on the age and type of kid. Kids at the most physically exhausting ages also tend to take naps which gives the parent a respite. You can also trade off with the other parent/other relatives if you're tired. Can't do that with work: Your coworkers aren't going to do your job if you have an off day.

The physically demanding part is in the demand for physical consistency.