IMHO, whether or not it is fat deposits or breast buds doesn't really matter so much. If your DD has a slight "bump" noticeable underneath her tee-shirt then I would have her wear an extra layer of clothing underneath her shirt for modesty.
Camisoles or sports bras work great for most girls but didn't seem to provide the "coverage" that I wanted for my DD. She is tall and thin and when she started to develop (at age 10) her breast development seemed, I dunno, high chested maybe? I found some pretty basic, lightly lined, sports bras at Target that seemed to do the trick.