My advice would be to forget about getting a computer to do the monitoring. You need to hire somebody to act as an editor / monitor / moderator. Seriously.

I was involved with running a game based web site for a large-cereal-producing-company for a couple of years. While some basic filters can help you, there is no avoiding having a human involved. We had somebody spend about 1/4 of there working week on monitoring this site - and it had few areas where generic text could be input.

Languages can be a real problem. We had a user running high in our site league for a couple of weeks whose user name was, if I recall correctly, the Swedish slang for... well... something not suitable for the under 14s the site was aimed at :-)

Rather than spend a lot of effort trying to prevent stuff getting up it may be more effective to have mechanisms to remove unsuitable content quickly.

The users are your best friend. Have a prominent area for feedback that goes to a real human who deals with problem quickly. Do not have a mailbox that somebody scans once a week. Somebody should be looking in there every few hours. Do this religously. Nothing will annoy a parent more than they're complaint going ignored for a couple of days.

Have a clear statement on the site of what you consider suitable behaviour, and that action will be taken to remove unsuitable content.

Oh yes. Find a lawyer. Talk to them about what you have to legally do if you're dealing with minors. Data protection laws, etc. are often different. Talk to them about the legal responsibilities if you're moderating content. Allowing moderation can, in some circumstances, make you legally responsible for the things that are said on the site (as ever IANAL).