I agree with the others that just about any dSLR out there can take the shots she wants. As for the lens, I'd recommend the longer the better, either the 18-250 (a great all around lens) or the new 55-300 (she'll still need the kit lens for everyday use). She won't need low light capabilities, and she shouldn't need lighting-fast AF, especially if she learns to pre-focus and pan.
However, please explain to her (again and again) that the camera won't take the picture for her. There are some great tips and examples posted here, and they're pretty easy to implement. If she spends just a little time and practices, she'll get some great shots that will give her and her son much enjoyment.
One way to practice is to go to a road or highway and practice taking shots of cars. This way she can try the pre-focus method, and get her panning technique down. It shouldn't take her more than once or twice to get to where she can get some nice shots.
Finally, I wouldn't "sell" her on any brand, especially because any brand
will work. If she's leaning one way or another, just let her go with what she wants and then help her learn to use what she's got. That's the key, to learn the equipment. Too many people expect that because it's an expensive camera, it should do all the work. That's just not the case, as I'm sure you know. Of course, if she's leaning towards Pentax, then all the better.