I'm using Nikon D7100 with 18-105 lens and nikon 50mm as well as?

I'm going to attend my son's annual day function in his school (like dance, play etc), want to take photos without flash, how to take photos and which should i take

Take the 18-105. That will give you good versatility. You want to be able to use a high enough ISO to let you use a shutter speed of I would say around 1/60 as anything slower you will start to pick up motion blurring. When you get in do some test shots, or you can set your camera to auto ISO with a shutter speed no slower than say 1/60.

I don't get it why people with no idea about the basics of photography by cameras like the D7100…
Anyway take the 18-105 when your far away from the person you would like to photograph, then you'll do like was said before and put the camera into S mode, set the shutter speed to 1/60 or better 1/80 and the camera will do the rest.

If you can move closer to the person take the 50mm. Its probably the f/1.8 50mm and a very bright lens. You will be able to shoot at lower ISO and faster shutter PLUS you'll get a nice bookeh, as it is a good lens for portraits.