Just to note, it's unlikely your niece sent you anything. A spammer spoofed her email address to send you unwanted garbage. On the Mac, nothing can install without your assistance (at least, not very easily). You must have clicked on something in the email. Unlike Windows, nothing in an email runs automatically on the Mac OS.
Open Safari's preferences and click on the Extensions tab. Remove any extension you don't recognize. Naturally, look for an obvious one that names Baidu.
eHow had these instructions. Don't know how accurate they are. I modified them some since the way they're written isn't very good.
Open the Applications folder. Look for a subfolder named Toolbars. Within that, open (I'm guessing another folder) named "Baidu".
Double-click "Uninstall."
Enter your username and password when prompted, and click "OK" to complete the uninstallation. The "Baidu Uninstall" window launches. The uninstallation process begins and takes up to a minute to complete. An "Uninstall Complete" message appears when the toolbar has been successfully uninstalled. Click "Close" to exit the window. When you restart Safari, the toolbar will be removed.
