why do they want it?
if they want to do electronic projects than maybe worth it, when i was kid i had to pay 200$ for a piece of cardboard with components on it, a "200 in 1 electronics kit", where you would connect components attached to springs, so you could hook in wires. it is only worth it if you use it, for me it was, and used it alot alot.
if they just want to learn to program then any old android cellphone will do.
if they want to add specific peripherals and need portability then pi is way to go, as Derstrom8 suggests, then again , with a little hardware knowledge/circuits you could do your own gpio header circuit and feed to PC via usb or something similar (but could be project on its own if not on the market already)
so far with what you listed: psu/mouse/keyb/screen are all found with pc,, but what if kids wanted to add a peripheral like this:
**broken link removed**