I don't have any kids but I truly believe I would not allow my children to have their own site under the age of 14-15 (minimum). At this point, I don't even think I would allow them to surf the internet on their own.
My main concern is that they are in a rapid developmental phase, and they need to learn gross and fine motor skills, engage in imaginative play, hand-eye coordination, free-form social interaction etc. If they miss out on those things at the right milestones, they can never learn them later.
For example, a report came out this year that myopia is often a result of lack of exposure to outdoor environments before the age of 6. So this means an adult can sit in a dark room all day and be fine, but for small children, it causes their eyes to form incorrectly. This is just one example of many of how the child's development can get derailed.