Looking for a way to lock down internet access

[bsharpe]

I have been handed a project that will need to meet the following critera. That is the use of an ASUS Eee PC, with either XP or Linux installed. These PC's will be handed to children so that they can access shared learning resources on the internet. This will be done through a VPN client probably CISCO that will allow connection to the learning resource. The Online resources are heavily filtered for childrens needs. so all internet content filtered in a way that matches the childs Age. I need to ensure that whilst connected that the PC has no other access to any other content given to them from the ISP connection which will be various ISP's.

In windows XP i could utilise an application formally know as sendforce a Novell End Point Secuirty component within ZCM to achieve this but my prefered method would be ubuntu Linux, However, i am having trouble locating a Linux option. or even a configuration that may achieve the same thing? i just need pointing in the right direction if anyone can help?

[bsharpe]

As an update and with some thought i could achieve the above with locking down firefox or the proxy settings in the desktop to use just one proxy. I have been giving some thought to the desktop as it is going to be critical to ensure that no files etc can be saved to the hard drive and that the menus will be configured to show only the apps needed. No access to terminal etc
I was looking at gnome but the menus are not very configurable so am now looking at KDE. I have changed the tag to edubuntu as that will be the goal of this project if linux becomes the system of choice which is what i hope.

But another way of locking down the internet side of things other than specifying a proxy would be good. ultimatly the ideal choice would be too look at the routers but that will be difficult due to the amount of ISP's invloved. The project is aimed for children in care etc.