I am looking for a Java tool/package/library that will allow me to force-kill a child process.
This tool/package/library must work on Windows platform (mandatory).
I had solved such problem in past using the same method you are suggesting here: use taskkill for windows and kill -9 for Unix.
On windows you can use alternatively WMI by either invoking script (VBS or JS) from Java or using one of interoperability libraries (JaWin, Jintegra, Jinterop etc.)
I do not think that this solution is so complicated as you are afraid. I think it is not more than 50 code lines.
Good luck.