(Video included - click more to view) It is unusual to have a breast reduction at age 12, but in extreme cases, it may be warranted. Your daughter is a minor, so she cannot consent to the procedure. As her parent, you can consent to the procedure, but she still needs to be onboard, and understand the risks and benefits short-term and long-term.
Breast reduction is more likely to need to be redone when performed at a younger age. Partly this is because Plastic Surgeons are more likely to leave the breasts a little larger on a younger woman, and partly because the breast may not be through growing.
The procedure can be extremely worthwhile when done for the proper reasons and with a good understanding of what it means. If this is something she has brought up as a problem, then obtaining a consultation from a Board Certified Plastic Surgeon is the next step. If she has not brought it up, she may not be ready.