I love this question. My DD loves the computer and has been independent on it since she was around 2: opening browsers and using drop down bookmarks to get to her websites. At first I was concerned about her getting into adult material and did look into software, but choose not to block her at this time. If you use Firefox there is add-ons for the browser, opening a port with only kid friendly approved sites, including starfall and PBS. The software even tracks where the user goes and if I remember correctly how long they spent in the site.
As for timing them, I thought we might need to do that because she was becoming obsessed with the computer but her interests changed and spent less and less time on the computer. Sometimes we go for days, even weeks without computer time.
