I think they should learn something like Z80 assembler. Get them used to the idea that computation on a von Neumann architecture occurs within a one-dimensional array of fixed-width, integer-addressed memory cells. When they are used to that, they can move onto things like C.

A perfect geek toy for kids would be, to me, something about the size and form factor of a cellphone with a QWERTY keyboard. There'd be a switch on the front such that in one position it functions just like a graphing calculator, and in the other position it behaves more like a KIM-1 computer, letting you craft machine language routines for it directly. There'd be a low-level monitor of sorts that would let you examine memory contents, assign breakpoints, etc. and maybe some simple I/O routines to load and store programs from the attached flash storage. Otherwise it's a 1970s era hobbyist computer, except it has loads of memory, a sweet color display, an ARM CPU and you can fit it in your pocket.