If you are trying to send from arduino B to arduino A, then what has the serial monitor got to do with it ?
The code you posted for arduino B looks reasonable, that should send 's' which is what you want to do.
I could suggest two things.
You don't need the carriage return.
You could add a delay after the Serial.begin() in case the device needs to time to start up.
You could connect arduino B to the PC to see if you get the 's' there on the serial monitor. If not, figure out why.
Make sure the TX and RX are connected the right way round.
If it doesn't work, try the other way round. I've had some devices labelled wrongly.