RE:How do you really protect them?
PC in the living room, set up an account for each kid, use monitering sofware or even set up a safe list on there accounts if they are realy young. Its not too difficult.
You can probably also lock out the Google settings page, so you can set them and stop them being changed. Im not 100% sure on that though.
Also, if you are worried about your kids knowing more about computors than you befor the age of 18 then you are probably reading the wrong site.