Don't worry about introducing them to open source until they're old enough to know why the source is important. Just show them cool stuff to do on the computer, which just happens to be using Linux. My family uses Linux Mint most of the time, rebooting to that other OS only when absolutely necessary.
My kids' first program? TuxPaint on pre-Unity Ubuntu. They both learned their letters and other stuff with it and had a blast with it. ktuberling is another one. My 11-year-old daughter has used Ubuntu, Xubuntu, and the MATE version of Linux Mint. She sees how much better the penguin runs than the manure from Redmond. What does she do now? Play Flash dress-up games from within Google Chrome and multiplayer Minecraft with her brother and I.