I signed her up for the Algebra I class that starts in October.

This was what I was thinking of doing after finding videos on the AoPS website:

Have DD watch the PreAlgebra free videos for what she hasn't yet studied (negative exponents and an exponent of zero, for example) and then she could go as far as possible in Alcumus (maybe working an hour a day) as a preparation for the Algebra I class in October.

I must confess that I considered getting the PreAlgebra textbook for her to cover what she hadn't had before (to do Aug-Oct), but I was hoping to save $50 bucks or so by using the videos and Alcumus. I see that MadeinUK suggests the book. I wish there were a good outlet for buying it used.

Another question for you, MadeinUK, I watched some of the videos for PreAlgebra and they seemed really good-- do you still feel the book would be superior to the video explanations? It's not too late for me to bite the bullet and order the book.