I have two PCs at home running windows 98 & XP with free downloaded AVG antivirus.
Can anyone recommend something similar to put on to my daughter's mac, before she picks up something nasty.

I think one of the main attractions of Macs is that you don't really need antivirus software :) 5 minutes spent googling around found nothing useful for free, which suggests there is little need or market.

I use norton on my mac.

I wouldn't bother with one, as there aren't any mac viruses.
I have my mac on pretty much all the time with broadband, and no problems whatsoever. MacOSX has a pretty hefty firewall included as well.

Most viruses now come in the form of .exe files and need to be executed in order to take effect, and of course the mac doesn't even recognise an exe file as valid!