I believe the only part of the head that doesn't grow is the eyes. The head becomes wider and longer as they grow so I'm guessing (as I've not really thought about it before) that the skull bones are all there just little in a small snake and as they grow they will reposition slightly into the adult head shape.
I over summarised, relative to the growth of a reptile the eyeball is pretty close to the size it will be as an adult when they are babies. I can't remember which book I read this in. I know in humans too a babies eyeball is about 70% the size of an adults eyeball. Pretty amazing to think we rely so highly on vision that this is required.