My sis-in-law is US citizen. Her parents' B1 application was denied in 2006 even though they have a citizen daughter.
The problem is the senator most likely would not reply to the request or send a useless letter. In case the senator does want to help, it's gonna be a long way to resettle a decision in favor of the applicants. So the point is they'd better reapply, which may be faster and easier.