If they have access to the computer with the same
user or Admin account that you do, change it.
Otherwise, they can use any software you can.
A different user account for them, with a login
and other parameters, limited privileges and
access to the system and internet limited, is
about all you can do.
There are some children-protective browsers;
those could be installed on a child's account.
see - ' mac os x children's software web browsers ' - google search:
http://www.google.com/#hl=en&q=macos+x+children%27s+software+webbrowsers&aq=f&aqi=&fp=azWfBBCvguA
You can also use most browsers to not allow content
from specific sites by URL; that control would depend
on the browser(s) in use and how they are set up.
The separate accounts for other users, where
you would also have to log-in to use the Mac,
would be a better idea overall; and then you
can limit access to the hard disk drive, and
the applications, and give them a limited access
and a few items in the Dock. And kid browsers.
This could also save you a lot of trouble should
someone decide to mess with the System or a
few of your important files on the hard drive. It
also would enhance overall security on the Mac.
Good luck & happy computing! 🙂