Don't show the parental controls app if there aren't any child users

If there aren't any child users, the parental controls app still shows up on the desktop. This gives it a high-profile position on the user's desktop, but it's not useful or interesting - as far as the user is concerned, it's wasting space and distracting from the things they care about.

The majority of systems won't have any child users.

It would be better if the parental controls app was only visible when there are child users present on the system (and the current user is an admin).