1. Nice, but slightly over-exposed; if you look at the skin detail in right hand around her eyes, there's almost nothing there. I would suggest taking this back into the pixel room and doing a levels/curves adjustment to reduce the highlights slightly. Good focus, cute pose; not wild on the angle, but that's a personal choice.
2. Ummmm.. about the only thing missing here is a number under his chin. The full-face 'mugshot' is rarely the most flattering composition, especially this close. I would suggest a vertical (portrait) aspect and step back a little to open up the image slightly and include all of the child's head. This one is slightly over-exposed, perhaps 1/3 stop.
3. As per #1 regarding exposure. Good catchlights in the eyes, but it would be better if they were less centered. Avoid cropping little bits of people, such as the fingers of her left hand. Cropping limbs is fine if you do it boldly, but avoid little bits, and cropping at joints.
4. Better from an exposure asepct, comments as per #3 regarding cropping. In this case, your background is a bit distracting. You have a very dark area behind the child's head and shoulders contrasting with a very light one below. Looks for lighter, homogenous backgrounds.
5. A classic pose which would have been better opened up just a little. Also note that 1/60th of a second wasn't enough to totally freeze the movement of the children's hands. I think the boy might be just a bit too young for this pose as he's not quite where he should be, but cute nonetheless.
The only thing is I'm not sure I like the very top of his head out of the frame in number 2, and and number 5 is a good photo but I'm not sure I like that the little girl isn't keeping her hands still during the shot...but I know you can't help that.