This is a timely subject - please forgive the shameless plug.
A friend of mine works for Next Step Ministries which helps rescue sexually exploited women. They just started (like last month) a new initiative called
Project Safe Home, which, among other things, is promoting safe internet usage in the home for families. This involves giving away for free the
Kids Wifi device - they only ask that you pay it forward, whether as a donation to the ministry, or some other means.
The main difference I can see between Kids Wifi and Circle is that Circle also handles wired connections.