The vagueness of "work" in this article is (IMO) a feature, and not a bug. Raising your children can be great work, to your own point. You can't half-ass raising your kids, as you well know, and it sounds like you get the most purpose and fulfillment from doing that. What PG appears to be arguing is that, whether you're doing a "great" job of it depends on: 1) how hard you work on raising your kids, 2) your natural ability, and 3) effort — and I trust that you satisfy all 3 of those preconditions, as a dad.
> The notion that people need to work through their 20s for this unknown future prize is silly and wasteful
First of all, to call it an "unknown future prize" is a bit of a mischaracterization. If someone were to argue that one ought to spill their lives into their career with no well-defined end goal, then I'd agree that it's silly and wasteful. But if you actually have a clear view of what a desired end goal looks like (it could be anything: a house with a yard in the Bay Area, enough money to retire early, etc etc), it can be entirely reasonable to defer gratification. Keep in mind that in the experiment, the child knows that there's a second marshmallow coming if they wait. Adults need to know what their second marshmallow is before delaying the first one.
Second of all, I don't think it's fair to make such a sweeping generalization for how other people ought to live their lives. The neat thing about PG's post is that it's sufficiently abstract that it can apply to anyone, regardless of what they consider "great work".