Screetch - No need to hand them out before you start, just keep an eye on your passengers, and tell them to let you know as soon as they feel ill. Like you say, it's probably a bit unnerving climbing into a light aircraft for the first time and being handed a sick bag
RedKnight - It did cross my mind, especially once I got back to Shoreham and had to wait in the overhead whilst ATC helped another pilot who was unable to find the field despite being close enough to disrupt the circuit traffic. I decided that it wasn't actually a medical emergency, and so the situation didn't warrant a Pan Pan.
Joel.