It's not Walt. We are not told explicitly that it is Walt. Perhaps it is Karl, whom we know was in Room 23 at some point. He could be classed as a "child" by Ben, in the way Alex is. Perhaps Ben is lying, to separate Karl and Alex. Karl's father may, like Michael, be looking for his son. However, it seems most likely that they are referring to Walt.
Although it's most likely they were refering to Walt, the writers may have deliberately left his name out to confuse people
There is a connection between this scene, and Walt's computer-conversation with Michael. That conversation was cut off when Walt said that he couldn't talk long, because "they're coming back soon" and concluded by telling Michael he needed to go "somewhere". Note also that this episode is called "the 23rd psalm".("The 23rd Psalm")