If we are talking children's book portfolio here, and I assume we are, then I think it is really important to tell a story with your art. I often see many artist's pieces with wonderful characters, beautifully rendered but nothing is happening.
Sometimes the character is looking at the viewer as if someone took a picture of him or her and to me, that's another mistake. If we are talking illustration for a book and not the cover (cover is different) then I think characters have to interact with one another and have to give the feeling that they are unaware that we (the viewer) are watching them. That's what I do when I'm working on a piece, I always have that in my mind.
A good portfolio illustration has to leave you wanting more. Has to raise some questions.. why is the character doing that? What will happen next? To me that is the key to a great children's book illustration.