One of the organisations mentioned is Code Club[1], who do exactly that - they run small after school clubs, with volunteer programmers running them (and with a teacher to monitor the kids). One of the founders, Linda Sandvik, spoke at HN London a couple of months ago.

They're always looking for volunteers, so anyone here with some technical knowledge who would like to help kids learn computer engineering, give them a look!

[1] http://www.codeclub.org.uk/