Sure it's nice to see toothy grins most of the time from children, but without expression in the eyes, your smiles, regardless of the size will look lifeless.
Sometimes when photographing children, or grown ups for that fact, I have found the easiest subjects to photograph seem to come prepared with a smile on their face. I use to think that their smiles were perfect and all I had to do was snap the picture. But the truth is often times when people smile like this, you don’t see it in their eyes. So, even when people, especially children smile in such a way, make sure that they are smiling in their eyes as well. If you don’t get that smile or excited look in their eyes, it will look phony. The trick is to engage them and their senses so that the eyes say they are excited. If you were to cover their mouth in the photo and just see their eyes, you should be able to see if they were happy and smiley or not. If their mouth says their happy but their eyes say otherwise, you will not have a pleasing portrait.
This post will not explain how to get those expressions from your subjects, I have written plenty of other material on the subject but again the trick is to engage your subject and the subject will smile and you will get a pleasing expression that both the subject and / or the parents will love. If you focus on the eyes with your camera, that is important, however it is equally important to focus on their eyes with your attention. It is the key to lovely portraits.
If you cover the mouth with your hands, you will still see an excited little boy.
