You haven't seen who's picking the oranges, have you? Yes, there is a large concentration of Cubans in the south of the state, but Florida has its share of Mexicans and Central Americans.
You do have a valid point, though, that to be a really meaningful comparison, a study ought to weigh how many years -- or generations -- the kids' families have been in the US.
No, to make it a "meaningful comparison", you have to compare like groups of people.
If you compare unlike groups of people, you are going to get unlike results [which is pretty much tautologous].