In my opinion, the most important aspect when it comes to photographing children is getting a natural expression out of them. I want parents to look at the image and say "THAT is my child!". Photographs that show an emotion resonate much better with the viewer and have greater impact. There are also a couple of other things to bear in mind:

Focus on the eyes. You may have a photograph, with perfect lighting and a great expression but if the eyes are blurred and out of focus, then the image loses a lot of impact. If the eyes are on two different focus planes, then focus on the eye that is closest to the camera.

I try to keep the eyes above the centre line, ideally in the top third or keep the face on one side. This makes for a more interesting and pleasing image. Composition is very important and I will be going into this in some more detail in a future post.

I personally don't mind cutting off the top of the head. If you look at many magazine covers, you will see that very often, the tops of heads are missing.