And a friend had the particularly unfortunate experience of his child accidently erasing the hard drive. Now, if you think about it, all of the challenges of an adult operating system still exist, with the added challenges of meeting the needs and abilities of children. Adults expect things to break. Children expect things to work. And they don't have the patience for a reboot cycle.
Children expect immediate responses and immediate feedback.
The important point is not how fast the computer runs, but how responsive it seems to the child. Children need a operating system that doesn't require literacy. That means developing an interface that doesn't require text. Children want to have fun. While they can briefly be forced to do something they don't like for their own good, they aren't likely to put up with something boring or tedious for years.