The Pyra should be able to charge off any cable plugged in correclty almost anywhere on the back, apart from the HDMI port (output only) I think.
XFCE is basically a clone of those windowing systems going back to the Zerox Alto (mouse interaction), Apple Lisa office system (icons and drop down menus) RISC OS (in its first release as Arthur 1.20, for the dock at the bottom), all of which was famously reused in windows 95. I wonder how your Niece would cope with Windows 8 though, which eschewed the idea of having a dock/start bar at all. Personally, on my daily use linux windowing system, I've also dropped the idea of having a dock. I've four virtual screens where I put my windows, and I rarely have so much on one screen that alt-tabbing between them is a chore. I do still run a vestigal start menu, but mostly start programs using shortcuts that I have on my initial panel/bar, or use the right click openbox menu to start apps (or start them by name from the terminal, if I'm already navigating using the terminal sometimes).
Our generation and younger might not need much in terms of paperwork to get starter, but the generation above mine often seems to like to start using a device by reading all of the paperwork they can. We shouldn't limit the device to specific age categories if we can avoid it.