I don't know that I can recommend a particular bag, but I can say that most of the teens I see with Coach bags (or other designers for that matter) tend to carry the smaller ones - demi bags for the most part. That's probably a good start - cute and not too expensive (well, relatively).
demi/pouch/wristlet, like most others said. i see lots of teens (my younger sister included) with signature stripe (especially in fun colors like punch and pale blue) and scribble. i agree, stay away from the legacy and more "mature" looking styles.