What makes these nice is the roundness to the subject, the three-dimensionality that she shows, dues to highlights and shadows. This is classic seamless paper + one light lighting. Using that light in that situation is what makes these photos look so good--it's the natural lighting effect, with actual shadows. It comes from using a more-classic size of light than the current fad of using a huge, 5-foot to 7-foot modifier.
Much lighting done today is coming out of oversized modifiers that belch out a big swath of soft,diffused, character-less light that wraps around the subject, bathing a huge area in soft, almost omnidirectional light. Lighting manufacturers love selling these products; these things are expensive! People easily succumb to the lure of the 5-, 6-, and 7-foot modifiers. I actually greatly prefer this older style of lighting for these particular shots. The shadows create a sense of realism, and the simple, light-from-one-source lighting actually is giving you hair highlights, and good hair separation from the light background, so I don't think a hairlight would have improved these. Adding another light would be another lighting pattern, different from this pattern, and the photos would look a bit more "lighted", a bit more contrived, perhaps.
Lighting styles are many, and varied, and they come in and out of fashion. This is a classic, simple, one-light style, with a lighting ratio that has a nice, moderately deep, yet well-defined shadow pattern. I LIKE this type of lighting. If you look at the lighting, by removing all the color, you will see what fine B&W images these will make...the washtub shot for example is a perfect example of using light to create realism, to create the illusion of a real,live person, a living little doll of a child. Moving to a modern, oversized modifier would make these look more current, and also a lot less interesting, a lot more shadowless, less three-dimensional and flatter. On #2, the one with her standing in the dress, maybe make the shadows deeper...I think the reason the foot was asked about is it looks a bit like it ought to have more shadow, be darker. #2 is a bit overly bright compared against the others, and darkening her forehead and face and all, by a little bit, would probably improve it.
Adding an accent light, like say a small metal reflector with a honeycomb grid and barn doors would add an air of artifice I think. A pure white background look would be another look as well, more "studio", and less "real". Of course, those are their own lighting patterns, and can be useful.