I have smaller breasts for my frame and have to be fitted for corsets all the time as part of my work so from experience, a corset is a good way to go because it will accentuate the waist line and where the garments style and fitting gathers round the front it will create more of a cleavage and make the breasts look bigger and the waist appear smaller! Always a pretty reliable choice.
Try to avoid anything too fitted such as lingerie with built in bras. The baby doll is a cute idea but it's not as fitted won't really don't anything for the breasts, depends what kind of feel you're trying to go for, subtle lingerie - baby doll, something a little more saucy - corset.
The point of corsets are to give you curves. Personally I wear the steelbonned ones and am an avid tightlacer so I can testify that even tiny girls will have more curvage. If she's not up for the real deal, there's cheap corsets with plastic boning that won't have as dramatic effect, but as long as its fitted it will give some shape. I'd recommend a halter top-style because it's the best for squishing up some cleavage.
They have all types of stuff at Victoria Secret most of it built for small woman some really cute ones. Ide say in general small woman have more options then bigger woman, as a bigger woman that's what I found.
Heck, all the cute lingerie is for small breasted girls! I personally am a double D and everything that is made for bigger chested girls looks like it was made for grandma. You should have no problem finding something.
Vicky C, Fredricks of Hollywood , etc pretty much any lingerie store has stf for small women, If she's the A or B cups they have padding. Trust she has more than a bevy of choices for herself before women with bigger breast have.
Baby doll sounds good... I don't think corset would look right.. well it could but baby doll would probably make her look a bit more curvy.