Barbie clothes are a great place to start if you want to teach someone how to sew.
Sewing a real outfit is often daunting, in terms of steps to completion, for a new learner and the chances for disapointing results is high. Kids want to get creative, skip a step here and add one there. It’s hard to let them freely express themselves when you paid $10 for a pattern, $40 for fabric and another $20 for notions.
It’s much easier to relax and let them enjoy learning to sew by having them learn by sewing things for barbies. If you have friends who are seamstresses you can get scraps for free. If your barbege really likes getting creative and isn’t disciplined about her stitching you can buy “no sew” fabric glues and stick-on velcro for closures.