Microsoft Windows supports two types of user accounts - Administrator and Limited/Standard. People with Administrator accounts can do ANYTHING on the computer. They can change the computer clock, install software, kill TimesUpKidz with the Windows Task Manager, and delete user accounts (including yours!). People with Limited/Standard accounts can do most anything they personally need to do on a day-to-day basis, but can NOT make changes that affect other user's of the computer.
To ensure the kids' can't bypass time limits, they need to have Limited/Standard user accounts. Only the parents should have Administrator accounts.
There are disadvantages to Limited/Standard Windows user accounts. The most onerous problem is that Limited/Standard accounts can't install new software. Some games (poorly designed ones, I might add), require administrative privileges to run and/or update. For these reasons parents will ask if TimesUpKidz can work if their child has an account with Administrative privileges. They answer is a qualified "yes" - you can set time limits on an administative account. But kids with administrative accounts, especially those with technical familiarity with Windows, can defeat the software in any number of ways. This is not a limitation of TimesUpKidz. It is simply a fact about administrator and non-administrator accounts in Windows.