Remeber to state the image specifications for each when asking for c&c. With that said, I think you have done a great job with a lens that is intended to bring faster glass for less money.
Shot two is great! The rest could use a little improvement. First, it looks as though you are using the on camera flash. Try not to unless you have to. Even if you are not, you need to diffuse this flash. It is to hard and harsh lines will (and they have) form.
Shots one and three suffer from the flash. One is too bright. Turn the flash compensation down (power of the flash). Three gives a real solid shadow line. There are diffusers for all types of flashes, even pop-up flashes. Just about all are inexpensive.
Shot four is a focus problem. Remember to focus on the subjects eyes, when shooting people, then recompose.
Thanks for the comments. I was not aware that they made diffusers for pop up flashes and atm I do not have a real flash just the pop up. I am still trying to get used to metering the light correctly and sometimes run into problems with strange lighting situations especially when trying to get good shots of a 2 year old that never sits still.