First, forget what the other said: SHOOT RAW! It is the best way for you to correct your exposure in post processing. As for setting the WB, if you dont have a grey card, just set the white point in the eye or a tooth of the subject then adjust a bit to your liking.
Also, as someone said, get a color calibrating device! Get a cheap one for 100$ and use a gamma of 2.2. Macs uses 1.8 but everything on the web uses 2.2 so you better that way and the gamma is inbeded in the file so there wont be a problem when printing.
Now technicalities:
- both your zoom and the 50mm are fine, depending of what you want: blured BG: use the 50mm at 2.8f or 4f. Too wide and the face wont be fully in focus. If you want super sharp pic, go with f11, iso100 at 1/125.
- position: place the camera at the eye level of the subject, it seemed a bit too high in your pics
- Always focus on one eye then recompose
- never have a BG the same color as the subject. 99% of the time, it yields bad results.
- I would have framed the pics closer of the subject, but that's just me...
Finaly, you have a very good subject! She is very expressive and cute so experiment!