Can they stop TimesUpKidz with the Windows Task Manager?

As long as your child has a Limited/Standard user account, they can NOT stop the software using the Windows Task Manager. They can kill the part of the program that reminds them (and only them) how much time is left, but this is self-defeating. When time runs out, they'll get booted off without any warning. They do not have the privilege to stop the part of the program that tracks time and enforces limits.