How to send Serial Data to multiple devices.

As others have said, RS485 might work better. However if you already have hardware designed for RS232 reception (especially if you don't have long distances involved), another option might be to have several RS232 transmitters, all transmitting the same data.

I don't see any problems with your serial protocol at all though, as long as there is no chance of a receiver getting confused about the start of a frame.