[kappaloris]

In my opinion you should not use apps as a teaching medium. Apps are not that cool in the end and while some hackers might build apps, building apps for sure doesn't help you become a hacker in any way. Hence, my recommendation:

Python, raspberry/arduino, legos, and a 3d printer maybe. For conceptually higher level programming: a turtle graphics framework like netlogo or pythonturtle.

Nothing beats being able to program things that interact directly with the physical environment, hence the hardware recommendations. The only reason why this teaching approach is not universal is that sometimes you might want to try things that require too much effort or resources to be done 'for real', like a robot fighting arena, for example. Turtle programming is nice because its syntax is agent-oriented. This way you express things like path finding, competitive behaviours (robots shooting lasers at each others, pew pew), and other things that are cool (not only for kids) in the most natural way.

Maybe your nephew won't be able to write bounchy interfaces, but if he can learn how program a robot to make it understand where it's located and how to outsmart other robots, he'll already have half a CS degree in his pocket even before going to university.