Conan Doyle wrote in this group of kids exactly because, at the time, it was a realistic and familiar sight to urban readers. There are lots of newspaper articles from back then (and if i remember correctly, even lots of Parliamentary tirades) about them. Over a certain size, they had become a matter of public order.

More recently, I remember Dublin city centre in 1988, when Ireland was still pretty poor, being full of kids; and Marrakech or Fes in Morocco just 10 years ago.