What about kids in poverty in the US? Are you sure they're getting a good education? Are they hungry when they go to school? How hard is it to learn when you don't have good nutrition?
In addition, they get for free perfect knowledge of English, work eligibility in the US and an extensive network of friends and relatives. Something that many immigrants can only dream about.
No it isn't, Nigeria remains a poor country since its population doesn't get proper education. That keeps them much poorer than for example USA. Poor people in USA can be relatively rich from a global perspective since they get so good education, you couldn't keep the wealth USA currently has if the bottom 40% couldn't read like is the case in Nigeria.
Before you write "food deserts", the data isn't as clear as you think it is. (The "researchers" ignored food stores.) More to the point, stores go to where folks will buy what they're selling (unless they're burned out or lose too much to shoplifting).