We got DS a netbook when he was about 4. Mostly so that we didn't have to fight over who was using "my" computer, or things being out of place on my desk after he had used it, etc. At that point he didn't really know enough to explore beyond his favorite websites I had bookmarked for him, so it wasn't much of an issue. Now he has learned to do web searches and sneaks onto my computer to create things in word and use the printer. I think DH is in need/want of an upgrade to his laptop soon and at that point we will purge the old machine and put all the kid safe stuff on it.
