I found this on a quick search;

Because the Windows 98 platform is not considered secure, the secure administrator account as we know it on the more current Windows platforms does not exist. While it is possible to add additional user profiles to a Windows 98 machine using the instructions below, remember - Windows 98 is not a secure computing environment.

Please see the steps below to create additional user profile accounts on your computer:

- Log into the machine with the existing administrator password.
- Click on the Start button.
- Now click on Settings.
- Choose the Control Panel.
- Double click on User names and passwords.
- Click on the New User button.
- Fill in the boxes.

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So you should then have two accounts - the admin (main account) and the second account, which you could call KIDS. you can set a password to let you only access the Admin account using a password, and leave the password field blank for the kids one, so when they boot up they just have to hit enter to log in.

From the admin account you *should* be able to select what objects they can and can't get to and use...

I could be completely wrong tho - been a while since I used Win98, and when I did it was all single user/single profile set up's....