Does a portrait of a person need to give prominence to that person?

If they are holding an object, in this case a caterpillar, why not compose the picture around the caterpillar instead of the girl? The girl is still there, just out of focus. But it's still a picture of the girl.

If you're photographing a child holding a ball, try a few shots of the ball with the child's hands on it. If it's a toy, try the same thing. There's no rule that says you need to focus more on the person than the activity they're engaged in.

You can also show prominence to a part of the body other than the face. Hands and feet are common, as are gestures:

I've posted this pic before. It's a pretty good example of giving prominence to part of a person that's not their face. These girl scouts were on an outing and were obviously getting tired and a little bored. I tried to capture that part of them in this shot. In doing so, I purposely chopped off their heads so I could focus on their crossed arms. Without their faces, it paints a more detailed picture of their mood.