This was such a fun project to do with my 5 1/2 year old. We've now got a permanent ATC station in the kitchen with precut cards, pencils, markers, pens and glue sticks. She continues to make these at a great clip.
For the swap, she tried many techniques including watercolor, collage, pen and marker, colored pencil and even cutting out functional doors, cabinets and mailboxes with tiny letters. I think the size of the cards gives her a feeling of security to be experimental that larger paper doesn't. The same is true of me, too, I think.
The flickr group for this project is chock full of cards from many of the 900+ children who participated. A huge thank you to Erin and Blair for organizing this wonderful swap.