A decent solution that offers lots of options (Score 1)

I think the key is to realise that as your child grows/develops s/he is going to need differing amounts of supervision. You want them to learn to be responsible "internet citizens" and that means taking care of themselves. But you need something that will protect them whilst they learn. http://safensoundmail.com/ is currently in beta but offers a variety of options allowing you to adapt to suit your child as they grow. Apparently you can put "beta" in the referral code to get a completely free account.