How to fix dents in butt cheeks?

Hello dear!
Thanks for the pictures and provided information as well.
Looks like you could be a good candidate for fat transfer, to fix your problem and improve your buttocks, and I think you'll get very good results. 
l recommend you to make an in-person consultation with a board certified plastic surgeon for a proper answer and to talk about your goals and anything you need to know.
Good luck :)

Without posting photos to determine if contour deformity vs just dents very hard to advise. If truly dents I recommend Sculptra injections under local anesthesia may need 2 sessions 6 weeks apart. My fee per vial is $500. Most need 5 to 10 vials. If contour deformities fat grafting like lipo with fat transfer or B B L is a real operation at higher fees. Seek in person opinions. 

It is very difficult to determine the exact size and shape implant you will require to best match your ideal butt image without an examination by a board certified plastic surgeon. Not just any board certified plastic surgeon, but one with many years of frequently performing breast augmentation surgery including different approaches, techniques and implant choices. This is because several measurements not to mention your breast characteristics are needed to determine the optimal implant size to obtain your goals. Without knowing these dimensions it would be difficult to make this determination. For example, the existing base width of your buttock will determine, in many cases, the maximal volume per implant profile that you can accommodate. Liposuction of your lower back and fat transfer is also helpful as an adjunct to implants. Using “want to be” photos however are useful if simply provided to the surgeon.