Which is absolutely fine for one person buying and using one RPi. However, that is not the stated intention of the device - namely to introduce kids to programming.
Given that (assuming the project is successful), the vast majority of these boards will be in schools being played with by kids and kids being what they are, I can pretty much guarantee that the first thing the kids will do is overvoltage the unit (whether out of maliciousness, mischief or just plain "I want it to run faster").
I really can't see the need of this 'feature' in the device at all - it's not something that will be used by the device's target audience and the very fact that it's there could cause that target audience some problems.
As for the "Well, it's your fault, you downloaded that image." - How long would it take one kid to insert an SD card and reboot every device owned by a school?
Personally, I like the idea of having the 'never overvoltage' switch, however this requires specific knowledge and deliberate action by the device owners. I also like the idea of needing to use a serial number to enable the feature - it means that one SD image cannot just be used on multiple devices, it must be hand-crafted.
So I say both - permit the permanent disablement of the feature and also ensure that if the feature is requested, the request is specific to the board in question.
Well as i said if you include maliciousness and mischief there are plenty ways to vandalize anything. The "I want it to run faster" is of course a valid concern, but as you would need root access or another sd card i think it should'nt be as big a problem as you seem to think it is.
Maybe it's just me but i think being paranoid about kids destroying everything is not the right aproach to take. But that is more of a sociological problem, and offtopic.
The switch to disable overvolting would probably be the best compromise if it is easy to implement.