It must differ person to person on the pain, because when I had my second holes done, it didn't hurt but for a second.
Even if the ears are played with, it won't necessarily get infected, IF you take the time to clean the ears AND earrings often. My mother did many things wrong in her parenthood, but she knew that when I made the decision to get my ears pierced, she was making the decision to be responsible for my health. It sure "stunk" being told all the time that I needed to clean my ears/earrings but by the second time around, I had the pattern down and my ears never got infected ... that I was aware of, anyways.
They are saying lately not to use hydrogen peroxide on the skin because it kills off cells (that's the information a friend got last year when she got her belly button pierced,) but there's no reason why one shouldn't clean the earrings themselves in peroxide, then rinse them with water before putting back in the ear. Buy a second pair of the THICK-POST earrings to switch out and let the last pair washed air-dry (it's more sanitary.)
Obviously, if the gun isn't sanitized correctly, an infection can develop, but there are precautions that can be taken, and the parent should be responsible for the child. If it's easier to be responsible when the child is an infant, so be it.