Someone who does not shower for days on end and/or does not wear deodorant ever. They will then most deffinately smell like the sewer.
Man, did you smell Adam? He smells like a Sewer Kid.
A child who arose from the sewers. While forgetting to put on deodorant, this child may have exposed his BO to the entire football team. The hair of a Sewer Kid fits the look and smell of the individual; A curly rats nest.
That is one smelly Sewer Kid... Oh wait that's Adam