There are many things you can do, but like with all security, you'll need to stay one step ahead of your users.
The start menu is, first of all, not critical to the OS, but a function of a program called 'explorer' that is known as a 'shell.' You can run a variety of alternate 'shell' programs besides explorer -- google the words 'XP Shell' or search on a site like download.com and you will see a lot of options. WWW.ASHTONSHELL.COM has a beautiful shell, very powerful, but you have to pay for it.
That said, here are ways you can hide/disable the start menu without giving up on explorer as your shell.
You can press ctrl+alt+del, go to task manager, find the image explorer.exe, and end the task. This kills the desktop and the start menu. When you are done, you can go back into task manager and choose File/Run, type explorer.exe, and you get your desktop and start menu back. If you load all the programs the kid needs, and then use ctrl+alt+del to hibernate instead of shutting the system down, you can pretty much avoid use of the explorer shell altogether.
Here are some less intensive solutions:
You can click on the top border of the start menu, and drag down to the bottom of the screen, and the start menu will disappear. It won't reappear unless he figures out how to grab the border, at the very bottom of the screen, and pull it back up. To further hide it you, can drag it to top edge of the screen, or over to one side.
You can get rid of control panel by right clicking start menu -> properties -> start menu tab -> customize -> advanced tab, then change control panel to 'don't display this item'. You can also get rid of a lot of other options here.
You can cut items right out of the start menu and paste them in some other folder -- that gets rid of access to programs. You have to right click the start menu and choose 'explore' and also 'explore all users'.
