There is no reason that a child should ever have to marry someone at such a young age. They have yet to experience the world and learn new things, then suddenly they are going to being someone's wife? They need time to grow and learn who they are before they can ever think about being attached to someone else. A child should never be forced into this type of situation and it is sad that in most cases they cant stop it from happening. I've read about situations like these and sometimes children think about suicide as a way out, and they sometimes go as far as doing it. A child should not go through something like this.
It's essentially giving a girl over to a grown man for her entire life without her consent. It's assumed that she will have sex with him, and in many of the countries where this is practiced she can't press rape charges against her husband. It's putting someone in a binding agreement without her consent. I don't see the difference between child marriage and slavery.
Human trafficking has to do with moving people around and selling people as trafficking. Depending on the context, child brides may be situations where a child is married to someone. Both are wrong, as marriage should be consensual, and a child cannot consent. But in terms of what the things actually are, they are different.