You only pay one time to use TimesUpKidz throughout your home, regardless of how many computers you've got. But be aware that there is no remote control ability (you must be sitting in front of a computer to modify time limits). And time for each child is not tracked across computers; TimesUpKidz works best when each child is assigned to and regularly uses a single computer.
To use TimesUpKidz on more than one computer, install the Free Trial software on each computer. And then if you've purchased it, type your product key into TimesUpKidz on each computer.