If you only have 30 - 40 minutes to work on the card, then I would definitely say you'd have to have everything ready for them to put together. I know it can take me hours to make a card, so I'm pretty sure it would take newbies quite a while to complete a card from start to finish. You would just need to have adhesives ready for them on the tables. I personally wouldn't use an image that they would need to colour because, again, I know I can spend over an hour colouring an image. A pretty solid image pre-stampedfor them would work or they could stamp it themselves (then you would need to provide the stamp and ink). Perhaps as you go a long assembling the card, you can explain the process and what tools they would use if they were doing it themselves.
Thats my suggestions based on what I might do. Good luck. Sounds like it would be fun.