What works probably depends on the kid as much as the language. Not everybody (me for example) is ready for c.
One thing that is important is that you can find an engaging project. If your child thinks games are dumb, then gamemaker would not be the best vehicle. It sounds great, though and I am in the middle of download.
I introduced my kids to perl a little this summer. We wrote a program that made Captain Underpants names (you have to read the book). I did the bit that transforms the names and they had to fill 3 hashes with the new names. My daughter caught on to the idea of scalars as nouns and that the ; at the end of a line is like a period. And they got to use a text editor. It's a start, but thats not programming yet. And we had fun with all the stupid names.
What I want to figure out is how to help them (and my high school students) make the jump to the point where they can solve simple programming problems themselves. It's a large step from filling in some blanks to designing and building working software.