TimesUpKidz is integrated with the security in Microsoft Windows, so just put a password on your Windows user account. The password should be tough to guess so the kids can't figure it out.
When you want to set time limits, you first must log on to your Windows user account and this can only be done by someone who knows your password. Then you start TimesUpKidz - no additional password is required for this step. Don't forget to choose Log Off, Lock, or Switch User from the Windows Start menu when you are done using the computer; otherwise one of your kids can sit down at the computer, start TimesUpKidz (just as you can), and grant themselves unlimited time, or worse, uninstall the software.