Now that my daughter is old enough to be into everything, one of her favorite games is "let's turn Mama's yarn balls into a tangled mess." Since this can happen in approximately 1/25th of a second (how does she do that?) and it usually takes me at least an hour to untangle the yarn afterward, I've had to come up with some strategies to head her off before she gets into trouble.
Most recently, I took some leftover styrofoam craft balls - you know, the ones you paint to make into planets for a science fair display - and some of my smallest balls of leftover yarn. I used a gluegun to glue the yarn to the ball, wrapping and glueing liberally at the beginning. I wrapped until the whole foam ball was covered and it looked vaguely like one of my real yarn balls, then I left the last few feet of yarn unglued so my daughter would have something to play with.
Whenever she makes a beeline for my yarn drawer, I remind her that those are MY balls, and she has her own yarn in her sewing box. As long as I don't trot them out too frequently, these keep her busy for at least 10 minutes or so. That's long enough to crochet a few inches, at least!