I've used this book with gifted 6th graders:http://www.amazon.com/Greek-Latin-Roots-...38577375&sr=1-1
I like the subject areas the material is divided into. Also, if your daughter likes learning how languages work, the way Latin derivations are explained might appeal to her -- the author demonstrates how one Latin verb can, through different principal parts (capere / -cipere, for example, versus captus / -ceptus), give rise to English words that a beginner might think were unrelated (capture, recipe, perception, etc.).
If your daughter likes etymology, she might find www.etymonline.com
useful. It's my go-to website for answering kids' etymology questions since merriam-webster.com doesn't give etymologies for all the words it defines. The explanations at etymonline are pretty thorough, but not too difficult IMO for a child with good dictionary skills (who knows M.Fr. means Middle French, for example).
If she likes online quizzes, here's a decent online game:http://www.prefixsuffix.com/rooty.php?navblks=1011000
Last, Michael Clay Thompson's Word Within the Word series has some very imaginative exercises -- much better IMO than the typical fill-in-the-blank and matching exercises in, say, Red Hot Root Words (which I've also used). The drawback of WWW, though, is that its etymologies are rife with errors, and I don't know whether Royal Fireworks Press has ever corrected them or published a list of errata.