BTW: I started programming in 4th grade, with printing terminals and acoustic-coupled dial-up mainframes. A few years later I had a then-equivalent of the RasberryPi: Dad soldered together a $100 Sinclair ZX-80 (likewise, the screen was your TV), and I programmed that thing darn near non-stop. Heck yeah the RasberryPi is "gonna be much use to a primary school student". Can't wait to get my kids going on it.
Whatever you answer, the cost dwarfs the device, and swamps its processing power.
So, we're talking about teaching kids to use some special kid-computer with software/tools they will never encounter ever again. What tools are these? A BASIC interpreter?
Linux costs 0, and runs on far less capable devices than the RaspberryPi.
"What Linux distros will be supported at launch? Debian, Fedora and ArchLinux will be supported from the start." (Raspberry Pi FAQ)
Been down that road, burned more times than I can count. I wish it were true, but I don't fool myself into believing it.
Load DSL (Damn Small Linux) on this, get gcc set up with a decent minimal text editor, and yes I'll start them with C++ from the ground up - akin to how I started, but with industry standard tools from square 1 and more introduced as they progress.