So I've done two now and learnt LOADS.
Firstly, I love it. It's hard work, and not the best-paid work I've ever done by a very long way(!), but it's really satisfying to make something for somebody and have them say "Oh, now that is nice! Thank you!"
I still don't know what sells well - it just depends who is browsing your stall. One of the very first cards I sold was one I was really unhappy with but the man who bought it said "Oh, that's just right!"
The survival sacks went down well - one lady today said she'd stick a fiver in it and give it as a "bigger" present, so I suggested that to two more people later in the day, and they also bought them 
Thank you, JM for those - I think they'd sell really well nearer Christmas too.
DS2 has been beautifully behaved both times - just fed, slept in the pushchair and beamed at everyone - DS1 has had two lovely days out with DH.
I'm going to take the cards I have left round local gift shops and see if they're interested in selling them, and I want to get my website up and running so that I can sell through there - loads of people asked if I had a site.
I did a LOT of personalising cards on the stall, and made loads while I was there too. I made a few from scratch - two anniversary ones today, with LOTS of writing on, which scares me, but I'm getting better at it.
Am shattered but happy. And I'm going to book the next stall date so that I don't let the momentum slide 
Thank you to all the A&C MNers who posted encouragement and ideas - MUCH appreciated.