Maily is a fun way for kids of 4 and up to stay in touch with their relatives by email. It's a free iPad app that allows parents to set up a list of email recipients, in the form of headshots. Your kid can create images by mixing photos, freehand drawing, and drag-and-drop skip art onto the screen, then send it to a recipient by tapping their headshot.
Paranoid parents can use a setting that shunts their kid's incoming email into their own inbox, so they can make sure grandma didn't send an off-color joke before forwarding it to junior.
The design and ease-of-use are excellent. I wish Gmail was more like this!