Looking at Computers for a Kid

So my son is actually pretty advanced, but he is still a child of not quite five years. I let him use my computer when I am supervising him and he does quite well. He does not read and write just yet, but he does know how to do a lot of stuff. My computer has a lot of important things on it, so I am looking at laptops under 300 dollars. I figure that if I find a really cheap laptop it will not bother me too much if something bad happens to it. In particular it will not bother me if I am thinking that it could have been my computer instead of a cheap laptop. I have a custom built brute actually. The graphics card cost me close to the sort of money that we are talking about. In all the computer cost over 1100 dollars just for what is in the case, and I had a friend of mine put it together.