I almost bought a Pi for my son when he turned five, but then I decided to hold off (he didn't seem mature enough to be trusted with it yet). I want each of my kids to have a device that they can use how they please—whether it be playing Minecraft or tinkering with the hardware (including destroying it—I broke quite a few computers growing up...).
What software do you have running on the Pi for your 3.5-year-old?