Overall, interesting shots, interesting post processing. Besides your niece, you have some earlier portraits at the bottom of page 3. And, the band.
B&H says your camera is unavailable. Henry’s will ship one tomorrow. Their kits have different contents. Did you get a flash? If you did, can you aim the flash independently of the camera? Do you have any lens other than the 20-50 f/3.5-5.6?
Your camera has an APS-C sensor that limits angle of view. Crop factor is 1.5. So with a 50 mm lens, the camera sees what a full frame camera would see with a 75 mm lens. But you still have a 50 mm lens. Some people think a 50 mm lens is fine for portraits, others think it is too short. Actually, portraits are shot with lenses of all lengths, even 4 mm, but the shorter lens, the closer you are apt to be, and the greater the distortions introduced. Forty years ago when film based 35 mm SLR cameras were popular, the 135 mm was considered a great portrait lens. With full frame dSLR’s the 85 mm is popular as a portrait lens, as is the 135 and the 70-200 zoom. Cameras with smaller APS-C sensors suffer because the smaller sensor crops, but the lens is the focal length stamped on the lens, and you need a shorter lens if you are going to fit the same image into the frame, from the same vantage point. Because you pull back to get the framing, or use a shorter lens, shallow DOF is more difficult to achieve with APS-C. I mention this because it is important to understand how smaller/larger sensors affect your photos, and what the sales clerk said may not have been very accurate. Moving from a full frame sensor to APS-C doesn’t turn your 50 mm lens into a 75 mm lens, it just shrinks the angle of view. I’ll leave you to do your own research and testing to gain a fuller understanding. I’ll also say that at the rate you are progressing, you will be running into limitations of your hardware.
The tip I would provide about your niece’s portraits is that even outdoors a flash can help. Sometimes you want full power and sometimes you want to dial it way back to 1/64th power. It depends on the look you are going for. Very low power will provide catch lights without affecting the rest of the shot. More power will fill in the shadows in eye sockets.With even more power you can overpower the sun and completely control your lighting. Being able to aim the flash independently gives the ability to bounce the flash off conveniently located surfaces, either walls or photographic reflectors. It is another area for you to research.