Photos 1 and 2 have too much top space aka head space; photos 3 and 4 have a classic amount of top space. In almost all instances, your camera's lens height is too high above the eye level of the subjects. The horizontal framing of the fifth photo makes little sense,and you've done a remarkably good job at resisting the single biggest temptation beginners succumb to,which is to photograph single-person, standing portraits in a horizontal orientation for no good reason.
Your last shot, the full-length shot of the littler boy is probably the best shot, overall, from a technical and aesthetic point of view. The camera's not too high, his entire person is shown, the background is mostly well-controlled and out of focus, and the setting helps to give us a good sense of just how big he is. If you would have taken maybe three steps to your right, the tree in the background behind him would have been in a better location,and also the brightly highlighted park bench reflection in the lower right hand corner might have been eliminated or at least made less-distracting. While it's a small thing, a bright highlight at a corner of a shot is kind of distracting sometimes. But his position on the bench gives us a sense of how big he really is, in relation to his surroundings.
These are not professional-grade portraits, but you're doing more right than wrong I think,and the kids look good in them for the most part. Overall, I'd say lower the camera a bit more,and watch the backgrounds as much as you can. Strive for relaxed expressions and avoid forced smiles. Flesh tones and white balance look pretty good overall, colors are not way out of line. The second shot, of the littler guy crawling along, could benefit from cropping a tad bit off the let hand side, and a significant amount off of the top. That shot once again, SHOWS an age, a stage of development, that makes sense.