>Thanks for an informative post. I already have three battery-powered flashes, two mains-powered studio flashes and an RF trigger set, so I'm covered there, and I must say proper flash photography is a lot of fun (Strobist and cheap ebay equipment got to me...)
That's a decent setup - my own kit is mostly from second-hand purchases as well so I can appreciate that.
>The background is plain ( e.g. white paper table cloth)
In general I would recommend avoiding white backgrounds - dark blue, gray or black is usually better unless you compensate for the reflection in your flash. More importantly the high amount of white in the picture will throw off the camera's sensors so you cannot even use them as a guide for exposure (in practise you almost HAVE to over-expose a white background to get the subject correctly lit).
>Would the purpose of shooting this in RAW instead of JPEG be that the e.g. 1/3 stop I'll have to adjust it in gimp will be better based on the RAW than the JPEG? Or would it be more that the RAW lets me get more creative in post-processing, playing around with curves and colors to turn the picture into something significantly different from what my camera captured?
Both. The adjustment in RAW will produce a better quality outcome. Now the amount of modification you want to do will vary. Many of my pictures contain nothing but RAW editing, while others are significantly adjusted (for example taking a portrait and trying to reproduce the kind of post production that Diana Holga invented) - it's nice to have the option even if you rarely use it.
Even more subtly there are things like pimples. Even for a picture I will barely be editing I would generally get rid of those, pimples are temporary blemishes and they are not representative of the person and have no place in their memories as far as I'm concerned.
I mostly do glamour and erotic-art photography, often with models who pay me for portfolio work and that also comes with expectations to show them just a little prettier than they really are. In those cases I would often create a layer from the background, blue the hell out of it (gaussian blur with 50 pixel radius in both directions) and then slowly adjust it's opacity down till I find that perfect balance between soft skin and impossible perfection (though one of my all-time favourite shots I deliberately pushed it up until the girl's face looked as plastic as a Barbie-doll and named the picture "perfectly flawed" to make a point).
So the degree of value you may expect from RAW shots really depend on what you do with your pictures. For the kind of scenario you describe - I would say the biggest advantage is that you get the power to make subtle adjustments to lighting. Studio lights give you picture-perfect setups but they don't really adapt well to the nuances of a specific shot. Depending on how your daughter was posed, what you want to stand out and remember in one picture may be subtly different from another - RAW gives you the power to make those subtle changes with little or no loss in quality.