Strange Keyboard Bug

So, me, my mom, and my dad like to play Minecraft together. We all have a MacBook each, but on basically any version below 1.7, the keyboard on the macs become randomly unresponsive. Everything else works just fine, though. However, I've got a custom-built windows desktop that runs those versions just fine. I have no idea why this happens, so please help! If it requires some kind of weird mod-thing to fix, or even a different version of java, I'll try it. Thanks in advance! :)