I would say gmail.com It's not for kids, but it's free, secure and you won't get any spam unless you give out your email to companies.
I do think that even if you found one for kids, it's use needs to be monitored by you regularly. Kids are VERY liberal with giving out personal information online (I know this because I moderate chat rooms). They will give out school names, phone numbers, town or city names, and emails at the blink of an eye and then the aim, yahoo, msn, Myspace, piczo, xanga stuff is a whole other ball of worms...
I think the safest email would be to have a family account that you can access for your child and he/she can read them with you sitting there and send to friends with you sitting there.