OpenDNS prevents Flashback infection, and if your machine has already been infected, it will at least prevent the malware from connecting back to its command and control server.  My next door neighbor, who has two teenage children like the OP, uses OpenDNS to keep his shorties from becoming pr0n junkies, and he and I use each others WiFi pretty regularly when we are hanging out.  As far as speed goes, I haven't noticed anything negative.  He has a cable connection and I have DSL, and he still has faster download speeds with OpenDNS.  The only thing that I actually notice is the filtering blocks access to a very broad range of web sites, some of which I have no idea why, but that is due to his strict filtering.  As far as malware/botnet security goes, OpenDNS will prevent DNS request to malicious servers; DNS responses containing IP addresses listed in RFC1918 will be filtered out.  Sites known to be vehicles for distributing malware or phishing are also filtered out.  It's a pretty comprehensive service, and it is probably worth a try for you.