Children can and do adapt to use adult operating systems, but they would far better be served by an operating system designed to meet their unique needs and abilities. I have watched several nieces and nephews struggle with adult operating systems, often becoming very frustrated. 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.