Okay guys. So here's the drama, and the rest of the story.
My mom calls me and she's just gotten off the phone with my cousin's older sister. It turns out, my cousin
doesn't like Juicy.

There was some miscommunication somewhere...we originally thought anything Juicy would be great for a 13-year-old, but of course, leave it to my little cousin to go against the grain.
So I bought this today instead, as some previous posters had suggested:
I had never seen it in person before...it's so cute! For some reason I thought the heart would be very large and clunky, but it's so small and dainty and sweet...perfect for a 13-year-old (hell, I'd wear it, it's pretty cute), and very appropriate for a bat mitzvah.
So after ALL that, I'm returning the Juicy necklace and giving her the Tiffany necklace. I love the fact that she gets her first piece of expensive "adult" jewelry from Tiffany and for her bat mitzvah, and I think it's a beautiful, versatile piece that she can wear forever.
And I figure if she hates it, whatever, she'll have a blast going to pick out something new at Tiffany!
So yeah. Thanks again for all the suggestions and advice! I appreciate it!