Cave Under the City — Harry Mazer

Nobody said New York City was easy, but for Tolley Holtz, it's home

Tolley Holtz would rather be exploring the city with his friends, talking about the Yankees, and discussing what President Roosevelt is going to do about the Depression than watching his little brother, Bubber. With his mother working all day in a garment factory and his father looking for work in Baltimore, Tolley's family depends on him.
 
But suddenly, things go from hard to unthinkable. When his mother gets pneumonia and can't leave the hospital, Tolley becomes the only family Bubber can rely on. Afraid that the children's shelter will split them up, the two brothers hop a crosstown bus and strike out on their own, up and down fire escapes, across rooftops, and into the cellars and shadows of New York City. Tolley will need all the street smarts he's ever learned to get them through the winter -- and to hold on to his dreams of having a family again.