[JTxt]

I'm voting up all the https://scratch.mit.edu suggestions. I was active in that community and with the developers early on. I wish I had it when I was little.

You can browse projects with the 5 year old then view the source and figure out how they work. Show that they can change things.

It's awesome that you want to promote creation over just consuming content like most of their peers.

Scratch your own itch with it too. Make a 3d renderer or CRUD app, fractal explorer, or game... they might like to see what can be made and what excites you.

They may get into the story and animation projects made by other kids first, that's good too.