How can I view Mac OS X Parental Control files with format .data?

These date-specific files are regular binary plist files created from a Core Data object graph. Open with Xcode 4 or Property List Editor (comes with Xcode 3), or any text editor after you convert it to XML using plutil -convert xml1 filename.data -o filename-xml.plist in Terminal. The content is pretty much useless though, unless you know how to load it again.

Much more interesting is /Library/Application Support/Apple/ParentalControls/Users/username/events.data

This contains the user-specific applications, web sites and chat protocols in a SQLite container format. Open e.g. using Base, other tools here and here.

The date columns are seconds since a date and time in early 2000. For me, comparison of the GUI value and experimentation showed the 0 value to be Jan 2, 2000, at 2:00:00 AM. Dates shown are May 18 according to the UI.

I suggest you focus on this file only; I believe the others are simply helper files for internal data structures.