The limitations being that they need a program like Net Nanny or Cybersitter or whatever (Proxy Server) to prevent them from going to web sites that they shouldn't get into. Also policies need to be set on the system so that they cannot add or remove software or mess up the hardware configuration. I've seen so many teens and pre-teens futz with the systems to the point that they are unworkable.
In any case, monitor the child, if they are drawn into the computer too much, they need to take a break. I limit my son to 15 or 30 minutes of computer time a day, but that is because he is under 3 years old and needs to do other stuff. His system is currently broken, and as soon as I get the replacement parts to fix it, he will be back up. No Internet access for him, just the Pre-School and Toddler games on CD.