Yes, Virginia, botnets exists and run unbeknownst to the owners of the machines. Is that the case here? Who knows. The mother sure doesn't and neither does the prosecution. Is it likely that the kids downloaded the software? Sure. It's also likely the machine was unknowingly compromised.
You can insist all you want about how the mother should watch everything her children do on the Internet (yea..right... don't let them go to the library), but in the end, the mother herself could have launched the virus without knowing it. Her only response when accused would be that it must have been a friend, since she is unaware of any other possibility.
And by posting to the root, you are posting in general, thereby welcoming comments to your 'opinions.' Next time, target your post if you are intending on a more personnal communication, yet still in public. Of course, if you expect no comments on your 'opinions' then may I suggest private replies.