Supposedly Woz and his best friend communicated via a wired intercom they stretched between their houses, using parts given them my nerd-friendly PacBell linemen. (Dude, I haven't read iWoz in ages so I might have mangled that story.)
One failure of the classic intercom is if the other person isn't there, no communication takes place. What I'm envisioning is an arduino-controlled system where messages can be transmitted to and stored at the remote site.
My original idea is to not have a keyboard, instead using keywords like Food, Sleep, Ball, Movie, Ball, and would be intended for kids who haven't learned how to read yet. Maybe there could be four words, chose by turning knobs, and you press a button to hit "send" -- and the receiving station shows those four keywords. One station could be in one kid's room and the other would be in the other kid's room.
(I tell ya, cell phones and laptops take all the fun out of walkie-talkies and intercoms.)