There are, they are very hard to use, and I can think of about zero legitimate reasons to do this. Depending what network you are on, this could be illegal as it is an invasion of privacy.
For reading IMs over a network with a Mac, I'd think you might need a packet sniffer of some kind. That is to say I don't know of a program that'll do this specifically, and unless these are your kids, I feel like this a questionably legal area.
Adium logs all the chats that any users on one computer. This won't however log people on other computers. Like other posters said unless you are logging your kids on the family computer receiving logs of private conversations would most likely be a illegal as an invasion of privacy.
For reading IMs over a network with a Mac, I'd think you might need a packet sniffer of some kind. That is to say I don't know of a program that'll do this specifically, and unless these are your kids, I feel like this a questionably legal area.
iChat CAN encrypt them. Your choice. Default is to not. You need a security certificate on both people's computers, and as most people are not technical enough to make and install one... you get the idea.