The first would be a parapet if it was on a roof or a raised walkway. I looked at Google Images for 'low parapet' but it all seems to be about roof parapets. I'm not sure what I'd say if I was with someone and wanted to sit on the/this . . . wall? barrier? Wall seems to be the best I can think of.
Railings, plural, or mbfhh's balustrade, for the second.