KidzMail looks good (via Gadgetopia).  It would have been even better if it was web-based and provided e-mail service as well like like Hotmail so kid-appropriate filters can be come pre-installed.  Even better, expand the functionality to include other activities including blogging, IM, games, trading, and, especially important, education.

Kid's perspective affects not the UI but the functionality itself.  Take a common functionality like spellchecker.  Instead of merely underlying misspelled word and a way to correct it, each misspelled word is an oppotunity to educate the sender AS WELL AS the receiver.  Instead of correcting it, the program can use crayon like coloring to mark it and then provide correction on the side with a line drawn to the bottom where definition and related information is shown.

A weighted dictionary can be used to teach words kids are not likely to know so new words can be introduced incidentally as well as via suggestions as the kid is writing a message.

Education is a under-developed country in the software world.  Although education can benefit greatly from correct application of technology, all we do is shove more hardware at them instead of coming up with better software and interfaces that widens the teaching opportunities.

Take for example, GameBoys.  There are millions of these things and kids are absolutely attached to them, yet there are very few educational software for GameBoys.  Even a simple software like electronic flash cards could do wonders to kids.  To do this, all one needs is a GameBoy cartridge capable of running Java (i.e. JemBlazer) and a means of communication with a nearby PC like USB, Bluetooth, or even Wi-Fi.