TI-89 Graphing Calculator. This is the perfect answer for you. It's portable. It's kid-sized. It's useful. It's what I learned to program on 20 years ago. The h

TI-89 Graphing Calculator. This is the perfect answer for you. It's portable. It's kid-sized. It's useful. It's what I learned to program on 20 years ago. The hardware and software architecture are positively ancient, in a good way, yet it's still sold and used today. Has its own BASIC. Can be programmed in C to the bare metal. There's an active hacking community and a good library of software and tools, but no commercial app store so all it's all hobbyist quality and it's feasible for your kid to contribute something meaningful that will actually get users, like a version of their favorite board game or something.