I help with a hour long 'computer lab' for my child's kindergarten class, and the teacher asked if I had any ideas for the last 3 classes of the year.

A big plus in usability is if the website reads to the student (with a contextual ? sign or something like that). This is critical for most of the kids based on their reading level.

Being a programmer, I would love to let them build (and publish!) a website, but I have not found a website builder that would be simple enough for a child. But that's just one idea.

Educational games are OK too.