$1/month/child at Zoobuh.com (Score 1)

We've been using Zoobuh (http://www.zoobuh.com/) for a year now, and apart from one outage that was caused by DoS apparently, it's been very good. Not free, but $12/year/child, with no maintenance from my end, is great. You setup their address book, and can set it up so that all email has to be approved by you first (outgoing and incoming). You can get copies of all their outgoing and incoming mail. This works for our little ones (under 7 years old) so far. Might not be the solution for teens.