Macs are cheaper. It's been successfully argued plenty of times. More outlay (although not much if you really compare like with like) and then they just keep working for 3 or 4 years without crashing, getting infected etc – no downtime.
Looks like Windows 7 has closed the gap aesthetically but I can't see the security being as strong as a Mac.
If your kids are creative, having Garageband and iMovie (as part of the initial cost) will be great for them.
The argument that no-one wants to write malware for MacOSX is such a load of rubbish. It's a huge target – mainly because of how secure everyone says it is – what trogan writing geek wouldn't want to infect it?!