I understand the thought with the kids. I have 3 myself. They never get anything like that in the mail. What about sending 1 item to each kid? If there is enough people doing this and everyone sends 1 item to each they would get more things? Does that make sense?
I've done swaps where say 10 people sign up... you send the host 10 exact items and paypal money for shipping a box back. The host waits until all boxes are in, then puts one of each item in the 10 boxes and ships back to each participant. While it works great for homemade products and the like, it wouldn't work as well for a kid's swap. Older kids aren't going to be "into" the same things as younger kids.