Depends on how you treat the word, but not here.
If you treat it as only describing poor women (or some other noun that is not masculine-personal), than the Nominative is "biedne" and Accusative is also "biedne".
If you treat it as describing either poor men, or just poor people in general (as here), then "poor people" include the possiblility that there are some males among them, thus the form has to be "biedni" (Nominative) and "biednych" (Accusative).
"Biednych" is in accusative, same if you add "chłopców". Confusion with genitive is reasonable because in masculine personal
plurals genitive = accusative, their adjectives look the same too: "(Nie) Widzę bogatych chłopców".
Other masculine nouns have identical genitive as above but their accusative = nominative (in plurals). "Widzę bogate koty", "Nie widzę bogatych kotów".