This made me smile. I remember trying to learn coding at 6 or 7 from my Tandy 56K Basic manual. I didn't get it but I tried all the same. It would have been cool to have someone there to help me understand.
It's a fortunate thing to have common interests with your child. I can't wait to have my daughters show even the slightest interest in coding. They are toddlers, so I have to just wait.
I was around the same age. My dad was a big ZBasic fan, and he had me copying BASIC games out of a book and getting them to run, on our Tandy 1000. My oldest is 5, and I'm trying to figure out the best time to expose him to it. With the entertainment-focused nature of computing we have now, I can't rely on the lack of distractions that were present with a non-networked computer with a monochrome display. Right now, we don't even have a regular computer for him to play around with—my wife and I have our own laptops—so I think the first step is setting up a desktop machine that the kids can fool around with.
I spent some time fiddling around with assembly when I was a teenager to run a TSR (terminate and stay resident, what daemons were called before they were daemons). It would hijack the screen when my little sister was playing a computer game and type out spooky messages like "Help me Sarah... I'm trapped in here" and then go back to the game after a few seconds as if nothing had happened.
Ah, how we laughed (after her nightmares stopped)...
I've had a lot of success with a Raspberry Pi 3 and a 15" monitor. My 3 1/2 year old learned how to use a mouse nearly instantly and can click around on the desktop.
The Pi is $35 and has HDMI out and lots of USB ports. It's pretty great.
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?
Well... the Kano distro is pretty good for "I'm gonna learn how to type and move around the computer". [1] They have a build-a-computer kit and are coming out with a screen soon? It's a little old for my son, though. It really wants you to type things which is great except he can't read yet.
A colleague of mine had a lot of success with Edubuntu. [2] Her six-year-old is a whiz at the command line. Again with the keyboard use, so not quite appropriate.
I'm still looking, basically. He likes clicking around and seeing that he's having an effect on things, but he usually ends up at YouTube, which isn't so great.
It's a fortunate thing to have common interests with your child. I can't wait to have my daughters show even the slightest interest in coding. They are toddlers, so I have to just wait.