First off, beautiful kids. They definitely have a model-esque way about them.
Now onto the pictures. I love them both, but I do agree with the chopped heads, but only on the lil girl. I think the boy picture has it all going for it, position, look, sharpness in the eyes. Good job.
I hear you on the copped heads! At first I was not sure but then found some who did not bother at all with it in other examples. next time i will not chop'em and then compare!
I like the shot of the boy best because both eyes are in focus, but in IMHO, both are way over cropped. And I'm not talking about just the heads, either. Sometimes you have to forget the rule of thirds, especially for portraits.
The pic of the little girl could have been truly awesome with that look.