Two languages should be taught. The first should be C. This gets kids used to playing with VB, JavaScript, and the like down to real machine level problems, like pointer manipulation, buffer resizing, data structure design, etc. You really learn how systems programming actually works with C, something that langauges like Java hide from you. And debugging in C is for real, chasing pointers and poking through data structures by hand. It's as close to assembly as you'll get without having to learn a specific machine architecture.

Second should be a language that breaks all the usual paradigms, like Common Lisp. You can teach OO in it, you can teach functional programming in it, you can teach compiler design in it, you can teach complex systems analysis in it, graphics programming, automated code rewriting and optimization, all sorts of stuff. You're abstracted away from the machine unlike in C, but you can dig deep and play with the bits if you want.

After that learning Java, C++, Perl, PL/SQL, COBOL, or anything else is pretty easy because the concepts in all of those languages have simple maps to either C or CL or both.