Not sure what I did wrong?

[alix-ivey.livejournal.com]

Hi, I'm hoping someone out there might be able to help me. I'm trying to make a smallish sculpture of a tree and I can't seem to make it stay together. I didn't want it to be solid clay so I molded it around a plastic gum container. I used air-dry clay from the dollar store. As it dried it it started cracking all over. Is it because I used cheap clay or because it shrunk around something unyielding, in which case what should I have used? I have very little experience with clay modeling but I saw a very cute wedding cake topper that I can't afford so I'd really like to be able to make it! Thanks in advance to any one who can point me in the right direction. :)