Rich said: Hmmm, I think that depends a lot on the 12 year old. Some 12 years olds can just go ahead and learn to use the adult field guides, no problem - so in my area, for instance I would introduce them to Symond's guides for the woody plants, and Newcomb's for herbaceous plants -though something like Peterson's for the wildflowers / herbaceous plants is a bit simpler than Newcomb's so may be a better choice. But those are all Northeast-specific. I'd suggest getting in touch with a local / West coast botanical society or botanical garden to see what would be more appropriate locally.
For a botany book (not ID) How Plants Work by Blackmore may be a good choice if they are a reader / bookworm type! I don't think it is a kids' book per se but it is in a simple and easy to read format, and doesn't get overly technical.
I hope this helps!