Working with kids can be a difficult task.  I found that kids under 2 are easy to work with while kids between 3 and 8 are the most difficult.  You have to try and get a real reaction out of them.  I took two of my own daughters up to the local park to try and get some great summertime photos.  My oldest daughter who is days away from being 10 years old was easy to work with.  I just had her run around and play and told her to ignore me as much as possible.  While my younger daughter who is 5 years old was a handful.  If she hear the clicking of the camera she was going to look over at me.  This means I needed to use a lens with range.  I used two lenses for my oldest daughter a 80-200 f/2.8 and a 50 f/1.4.  Being that I am using a Nikon D7200 the 50 f/1.4 with DX crop basically replaced my normal 85 f/1.4 which is the lens I really love to use for portraits.  For my younger daughter I needed be fast to focus on her to keep her entertained we used a bubble machine.   She was quickly playing with the bubbles and forgot about me for most of the shots.  We used a 80-200, a 17-55, and the 50.   Take a look at these lovely photos and let me know what you think.