There are tons of different icons of this type, and sometimes those icons mean to emphasize certain things, but in general the Theotokos and Christ-child icon means to depict the following:
This is an icon, primarily, of the Incarnation, the second coming of Christ. This type of icon is found on the iconostasis (icon wall) in front of the nave of an Orthodox parish, that separates the nave from the altar, and is balanced by the Christ Pantocrator, that depicts the Second Coming.
Christ, in true Byzantine form, is not depicted as a child, but has a mini-sized adult, which means to show he is not an ordinary child, but that he is fully God incarnate. He also has the special Christ halo (or "nimbus") that are found in most Christ icons these days.
The robes of the Theotokos generally has three stars on it, one on each shoulder and one on the head, proclaiming her virginity before, during and after the birth of her Son (this is also the norm for all icons of the Mother of God). Some types of icons (the "Directress" type) show her free hand motioning to Christ, pointing the world to him. Other types, usually called "Tenderness", shows the Theotokos and Christ-child sharing a loving embrace (their cheeks usually pressed up against each other), showing the intimacy of their mother/son relationship, confirming the full humanity of Christ.
I hope that's helpful, in a nutshell...