First, Crayola makes at least two popular kinds of "clay" --Model Magic and Crayola Air-Dry Clay. Model Magic dries hardish I think and doesn't do really fine details very well; Crayola Air-Dry Clay dries "hard" and will do better on the fine detail. I'm assuming you'll need fairly fine detail when making miniatures like "charms."
I'm also assuming that by "charms" you just want to make shaped (or molded) items, and not do all the other things that polymer clay can do. So if you can't afford to use polymer clay** I'd suggest that you use an air-dry clay that will do fine detail and that's nice and smooth to work with. If you want a homemade air-dry clay, that would mean bread clay (or even salt dough clay) in my opinion.
If you want a purchased air-dry clay, that would mean Creative Paperclay, or a "cold porcelain" clay, or perhaps Crayola Air-Dry Clay (or the pre-colored Makins or Hearty clays if you could find and afford them). Let all of those air-dry naturally (rather than heating) for the best results. You can read all about those air-dry clays and also sealing them (which will be necessary) in my previous answers to these questions: http://answers.yahoo.com/question/index?... (ignore the 2nd paragraph) sealing: http://answers.yahoo.com/question/index?... ...and more about making "charms" if you're interested: http://answers.yahoo.com/question/index?... http://answers.yahoo.com/question/index?qid=20120116075155AAlfxEl http://glassattic.com/polymer/info_letter.htm (click on the category "What All Can Be Done With Polymer Clay Anyway"?) .
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