1 and 2 are underexposed. You need to learn what exposure means, so you can understand how your camera calculates it. Both 1 and 2 have un-even lighting, which is to say that there is a brighter spot in one area of the photo and a darker part in another. The camera sees the bright spot, then sees the dark spot, and tries to find an average for exposure. What happens is, either the bright spots end up being too bright (blown highlights) so that more light can be brought in to expose the darker areas, or the camera lets in less light to expose the lighter areas, which causes you to lose all detail in the darker areas. Get it so far? Great.
Composition is better on #1 but.... *shrugs* like all of these, they're snapshots, with better attention to technical details they can live nicely in an album but the rest of us aren't going to appreciate them much.
This means that you, as a photographer, need to be able to adjust the even-ness of light coming into the camera - either make the darker spot brighter or the lighter spot darker. You can make darker spots brighter by using a fill flash (get a SB-400 or SB-600, look them up on Amazon), or make the lighter spots darker by using a graduated neutral density filter. Using the filter isn't recommended though for hand-held work because when working hand-held, you need a quicker shutter speed - putting less light into the camera would require you to increase your shutter speed and throw your camera on a tripod (GND filters are generally used for landscapes where the sky is much brighter than the land part of the landscape, for example). Also, don't try to fiddle with a filter when what you really want to do is just pick up a camera and shoot.
The composition is also wrong on #2 - if your son is going to face the left of the frame, then he should have space on the left of the frame for him to look into. Otherwise he's just looking into an empty border.
Un-rotate 3, adjust composition to put their faces and torsos in the lower-right of the frame and more space in the center. It's nice to see that you got down on your knees for this picture. Think about the "trails" in the picture (the areas with no grass due to tire wear) - in photography, these usually lead to something. What's it leading to here? And his expression is nice but her's is a little pouty.
4) Harsh, harsh, harsh direct lighting. Oh God. He's even got a little red-eye swimming in the back of his retina. See about using off-camera flash - watch this starting from about the 6:00 mark.
YouTube - Strobist Preliminaries
It may be about a bowl of fruit and not a person, but you can see how light can be much more flattering when the source of it (the flash) isn't shooting right in their face.
Again though with 4: see: composition, see: facial expressions.
Just because you have a little bokeh (background is out of focus) doesn't mean you are done.
And with all such threads: for those of you who say "I love it", stay away from C&C threads. If you don't know what you're talking about then you're not helping the other person improve, and you're not helping yourself.