They're all only as good as their virus signature database, which means you're always vulnerable to an attack they haven't dealt with yet. The best defence is to pay attention to what you're doing and heed the warnings that the computer tries to give you about installing untrusted software. And don't give children admin privileges, 'cos they NEVER read the warnings.
There are a lot more viruses around these days for Mac too - they're not inherently safer than (modern) Windows, just that there's more bang for your buck authoring for Windows.
IMO, of course.