If your current goal is to try and stimulate her affinity for math, you may wish to go with resources which she has expressed some amount of interest in... the story-telling style of Life of Fred.

If your goal is different, such as getting her to perform at a specific level, and/or to like a specific resource because you like it (AoPS), possibly with an underlying thought that high math achievement and/or experience with AoPS may be consistent with stereotypical "giftedness"... that may be more difficult.

Because you mentioned online resources, there's also the Khan Academy review of common core math. This may also help you understand what level she is at, according to common core standards.