Employment options are limited far more by other factors than cognitive impairment brought on by lack of sufficient nutrition.
In my experience, now that I'm working back in an environment that includes "general labor" the three main limiters are:
1) Showing up for work
2) Actually doing work when you're there.
3) Showing up for work reliably.
Back when I was in IT, yeah, cognitive abilities affected ones ability to get into that realm. But not so much with turning bolts and picking weeds. These are things a child can do.
Simple fact is people don't want to work. When Arkansas and Georgia enacted strict (and I would say stupid) illegal immigrant policies farms couldn't find people to work the fields. Jobs paid $12/hr in Arkansas and $17/hr in Georgia. People would not do them. They're lazy. They'd rather collect unemployment and welfare benefits.
And, De Selby, before you give me any crap about that work being too hard for the pay: I'm out there doing it. I prefer that work over sitting in an office writing code.