So, this will be very nitpicky, but I think that's actually what you are looking for so I will try to do well...But first -- 1&2 look pretty good, 3, as you know is the weakest. I think you wanted nitpicky so I'm going for it; if not --- well, I apologize...
1: The way his right arm is sitting, it is giving an odd effect of the arm being disproportionally short. Against that you have his left arm looking extra long by the way he has it stretched up to reach out on the top of the bench. This would have been a fine pose for an adult, or even a teen; but his body doesn't fit it yet so it has him struggling to position himself as you wanted. Maybe a more natural position for him would have helped the look.
You could possibly have pulled back some of the high lights from his shins, they look a little hot to me. And maybe just a tad of lightning of the shadows on his face could have been a nice addition. It may just be my eyes, but I think you actually short focused just a tad on this shot. I don't think you quite got all the sharpness out of his eyes that you could have.
On the positives, it is obvious by his head position and smile that A: hes a happy kid which speaks volumes; and B: hes done this a time or two, lol. Great facial expression. The background and his out fit worked out perfectly especially for an impromptu outing, sure the writing is distracting; but it IS what kids wear so can't really fault that too much on this type of shot. I also think you did a great job finding great angles in this and the other two shots. Now after reading that, this is just purely nit picking, it IS a good shot!
2: My first thought is tighten up the shot. I feel like I'm too far away. Having said that, when you look at the shot in a bigger format (ie large at flickr) it works better. This may just be one of those shots that doesn't work well small but is perfectly fine as a larger print.
Again, I would consider a little dodge on their faces to fight off some of the dark spots. Maybe see what you could do to pull out the polka dots just a little more on the front of her shirt. The reflection of light off them kind of gives the appearance of over exposure but I think it's something you can fight off. As for his left hand and the flyaway hairs -- maybe if this was a couple of contemporary adults, with an urban sheen then you would want to consider them issues; but I think they both fit these subjects perfectly.
I would suggest, however to clone out the white sign in the background. You could try lightning the black frame of the bench ever so slightly just to bring back some of that detail to add a little more depth to the photo. Again, my eyes are having trouble focusing on their faces, but idk if its OOF or PP'ing.
#3 - This shot is odd... not the shot itself, but odd in that while I think as is it is the worst, I feel it is in the least need of help and could have easily been the best. I really like the pose, the composition (minus the minor elbow clip on the right), the angles, every thing. Just reflect or flash some light on his face, then if you wanted to you could desaturate his cheeks just a tad or leave the rosey reds... I think either could work. I guess you could have tried to get him to tug his shirt down a smidge where it rode up, and again, solid shirt vs text would be nice but it is what it is. Having said that, I know you didn't come planning on a shoot so I can't really say much about the lighting to be honest.
All in all, I think they are all 3 good shots and show your talent for seeing lines and picking good shooting angles. I think you did a very nice job, and have no doubt that if you had been planning on a more serious shoot you would have pulled off an even better set of shots. These shots really made me stop and think about the angles I use when shooting, I think I (and probably a few of us) too often shoot strait on, and merely turn the torso or body a tad instead of leaving the subject more natural and positioning US at the angle. Great job showing how to just that!