1.what does "a public rest room"mean?
It means a public bathroom.
2.what are A and B talking about?
They are talking about where the public rest room is.
3.why does the man want to go to a public rest room?
Because his son needs to wash.
4.where is a public rest room?
It's in a department store on main street.
5.where is a department store?
It's on main street which is on the coener of Main and first Avenue.