Here is a quick example of how a simple improvement (increase the fill and adjust the exposure somewhat) might look.
There are still some composition problems that have been mentioned by others. It might be possible with many of the shots, if starting with the full size image, to add considerable fill after the fact in post. But, naturally, it is better to do it in camera.
Since you were using Fill flash, your camera may have a control to adjust the flash power for the shot. If you pumped it a bit higher, then that would help. But keep in mind that the photo as shot seemed a little dark, so you might want to work on the exposure. One thing to remember is that when photographing dark skinned subjects, you need to adjust the exposure from what the meter says since black absorbs more of the light.