Depends what they know. Are you teaching C, or programming-and-C?
I've seen very little success with the latter. C is simply not a very intuitive or forgiving language. I haven't seen students been thankful for starting with it, though I've seen students frustrated with programming for it.
The ones who are going to stick with programming will go out and learn C in their spare time, anyway. There's no need to push it on them first.
If you're just teaching C, and they already know pointers and arrays, then teaching how pointers and arrays work in C can be done in one lesson.
