I've got to correct myself and discourage VNC if that is all that is being tried.
VNC is sort of the second step in dealing with kids online. You should really have filtering established first. There is no way that you can constantly monitor every moment that the child is online. VNC only displays live use of a computer. It does not record. Consider a keylogger or chat logger to review previous activity, also google-able for OSX. iChat has a built-in option to save all conversations.
Supposing that you have a 40 year old stranger come to your house, do you seat him in the living room or send him to hang out with your daughter privately in her bedroom? Keep her computer in the living room and there will be no need for VNC. You might even promote better social behavior.