To my mind captions "oni pływają", "oni chodzą" and "dzieci biegają" have two meanings. First one is that they are doing this activity at this particular time, it's like: they are swimming, they are walking (they are doing it right now) Those captions can also mean that they swim, they walk, so it's like in present simple(this is the activity they usually do or they are able to do). "Oni płyną", "dziewczynki idą" and "kobiety biegną" have only one meaning: they are swimming, girls are walking, women are running. Here we don't have this second option; I hope it'll help you a little ;-)

Something like: oni/one chodzą - they walk, they go (focusing on the action; repeatable action; in general) "Dzieci chodzą do przedszkola." (generally) "Nie ma jak przejść, więc chodzą dookoła." (repeatable action) oni/one idą - they are going (focusing on the target; to a certain location; right now) "Dzieci idą do przedszkola" (w tym momencie) "Nie ma jak przejść, więc idą dookoła." (right now) oni/one biegają - they run, they are running (focusing on the action; in general) "Dzieci biegają dookoła domu." (action) "W wolnych chwilach lubimy biegać." (repeatable action) oni/one biegną - they are running (focusing on the target; in to a certain location; right now) "Dzieci biegną do domu." (target) "Biegniemy na autobus." (right now; target) Try writing some sentences with these or similar verbs.