A reasonably straightforward way would be to whip up a pair of shell scripts. One of these for locking the computer down, and the other for opening it up again. You could then put these scripts into cron so that they run at a given time each day, or you could run them manually with a double click.
Now, I'm sure there are easier ways around, but if I put this up on the board, we're bound to get 3 or 4 people telling us ways to make it simpler.
I'm going to assume that you have an account for yourself and an account for your son. Make sure that these are set up so your son doesn't have administrator privileges.
Then, take any apps you want to be able to lock (lets say IE and NetScape for now) and drop them into their own folder within apps called "Lockup".
Open a terminal, and set yourself as the owner of Lockup. (chown myaccount /Application/Lockup )
Now, you'll want to open your favourite text editor (not word processor, we're saving plain text here!) and enter these scripts.
File sys_lock.sh:
#!/bin/sh
chmod 700 /Applications/Lockup
echo "System is locked."
File sys_unlock.sh:
#!/bin/sh
chmod a+r /Applications/Lockup
echo "System is unlocked."
Now, from here you can see how you could expand it to include other folders or files as well, just by doing the same steps.
If you are a little afraid of the terminal, you could simply move the apps into a directory within your home folder when you want to deny access to them. This would mean a little fiddling around, but should work. The advantage of the scripts is that you can put them into your /etc/crontab file to have them automatically run at given times of the day (cron will also account for you leaving the computer off or sleeping, too, so no worries there).
