Think of the hundred of millions of kids who've been hooked onto Microsoft's products. Why shouldn't kids have a choice?
I agree with your last sentence though. One single device tied to a single OS is a bad choice. I'd rather give them a laptop that dual-boots Linux and Windows. That at least gives children a taste of the real world where more than one OS is used and learns them how to use both.
With chrome you can develop for the web which is the most open platform that works everywhere. And if you want to develop in native, you can install crouton and do whatever you want. Chromebooks are not locked in or "jailed", they all come with a dev switch that is documented by google and encouraged for anyone who wants to use it: http://www.chromium.org/chromium-os/developer-information-fo...
This is the way it should be in my opinion, the possibility to mess up with your device should not be enabled by default to avoid confusing noobs. But, if you're a tinkerer, it should be as easy as pressing a few keys and the process should be duly documented by the manufacturer and it should not breach the warranty. This comes from a happy pixel owner who enjoys hacking on Golang, js and even ruby among other languages on my chromebook. Best environment I've had after 10+ years of ubuntu and some osx :)