Alan Kay's evening talk is entitled Learners, Powerful ideas, and the $100 Laptop. He says that he's never found the right order for the ideas in the title.
Computer companies in the 1960s thought Moore's law meant that they'd get higher margins. Web presses are amazing pieces of technology, but when you look at it, you don't see anything that tells you about how it changes things. Similarly, looking at the DynaBook in 1968 doesn't tell you the most important things. Thus, the idea of a $100 laptop isn't important because of the technology.
Half the price of a typical laptop is the marketing and distribution. Get a non-profit and drop that. Half of the remaining cost is Microsoft, or more generally commercial software vendors. Free and open source software more than adequately covers the computing needs of most people, particularly children. The fact that there are $122 DVD players says you can build a $100 laptop. The cheapest hard drives are too expensive; so use flash memory.
One big problem is the grey market. They'll be diverted from children unless you do something to protect the laptop. A few ideas: an RFID card keyed to the specific owner helps. The device is networked, so the owner of the device has to log in every few days to get a token to keep it working. The color (green) helps. The child's picture could be embedded in the plastic case.