Natal (the province in which Durban sits) was a British Colony from 1843 to 1910, and they brought in Indian slaves to work on the sugar cane fields surrounding Durban, all the slaves were forced to speak English. The legacy of which means a very high concentration of English speaking population.
The U.S. Census does tabulate actual urbnaized areas, which often differ greatly. However, they bafflingly break down a few solidly continual urban areas such as Los Angeles and San Francisco into several urban areas, which leaves an underrepresentation of overall urban population. Go figure.
Not just kids. Saying "ready?" When you are about to leave and waiting for the other person, is normal and used as much as the dutch version by adults. "Nice" has replaced its counterpart. "No way!" is normal. The list is rather big. So I'll stop now ;)