You don't WANT a big old head for a costume for a toddler in any event.
Use a hoodie for the head.
Felt whit triangles lining it will be teeth. Sew (or buy) little round pillows (white satin, or yellow or green) for the eyes and use fabric paint or sharpies to draw the vertical slits and coloring if they're white. Use some "cat ears" and sew the ears facing the shoulders (instead of forward, like for cat ears) and sew the eyes to the ears so the eyes "stand up".
I would use a purple hoodie as the base, and then use either felt or satin to create the stripes and belly color. Purple sweatpants as the legs (ditto on the stripes). I would PERSONALLY attach the tail to the hoodie (makes it easier to sit, go potty, or change pullups).
By making the costume out of sweats you ALSO get more than one day's (or one week's) use out of them. Just detach the tail (or not) and they can be worn throughout the winter.
If you have a problem finding purple sweats the University of Washington school colors are Purple (and gold... but mostly purple) so it's easy to get PURPLE purple kids clothes. Just cover up (or remove) the school logo.
http://www.ehuskies.com/COLLEGE_Washington_Huskies_Sweats... or this for a warm hoodie/sweats of like this for thinner material
http://www.kidfanatics.com/COLLEGE_Washington_Huskies_Kid...