But you're right, they can prevent damage only after the fact.
The firewall can help prevent bad stuff from being put on your computer in the first place.
Kids love the Internet. And the spammers and hackers know this. Extra vigilence is needed. In the past, some of the more popular peer-to-peer software was loaded with spyware, keyloggers, etc.
Better safe than sorry.
FWIW, I have worked on computers that were so infected with viruses, spyware, ad popups, keyloggers and other nasty stuff that the computer would barely run.
Preventative and defensive maintenance is the only way to go.
Appreciate the help bigtime. I have some work to do tonight!