Tocomail is a site/app I recently covered that is ideal for having kids email in a safe environment. Not only does this site have a built in filter for catching inappropriate words but it just recently updated their app w/ a bully filter which looks for patterns of bullying and then quarantines them for parents to look at. This is just another safety feature for parents like controlling the contact list as well as viewing all emails that are quarantined.
I highly recommend checking out Tocomail by clicking here!!!