I was going to do exactly this when my son started getting old enough. In the local school system as low as first grade they have assignments that are online. I would like to have a controlled environment for my son to be able to learn boundaries and what is or is not appropriate.
My plan was to build my own proxy server, I may still do that but services like this seem to be a better solution.
I will see what is available in the next 2 - 3 years, a lot can change in that time!