Comfy and old desktop (Score 1)

First of all. I see a lot of mocking from the other replies here. The way I read your question, you're not intending to use this as a nanny for your son.
So I'll just tell how I have done it. Instead of a Eee or Eee-like thingy, I have purchased this nifty "keyboard": http://www.comfyland.com - I have used one of my old desktop machines for this.
The computer is placed out of reach, only I and my wife can access the power-button. I use an 19" LCD which I have wall-mounted and all cables go through a hole in the wall to where the computer is placed. The comfy-keyboard can take a lot of beating. I bougth it for my 1st-born which is now six, currently my 2-year old son and 1-year old daughter are fighting over it.
And no - before I get flamed. This is not the only stimuli my children get. We read books, we listen to music, we enjoy outdoor life.