The more I read through these responses, the more I'm leaning towards the idea that these kids are resistant to practice because the teaching method is, for them, every bit as inappropriate as a normal classroom setting for math or reading. Seems rather self-evident when you phrase it that way, doesn't it?
I think I'm going to revisit the approach for my DD and guitar. Rather than focus on repetition and skill acquisition, I should just focus on teaching her the concepts, then let her run with them however she likes. After all, that's the learning mode I adopted for myself.
