Teaching children to use computers is pretty darn vital these days.  Everything seems to revolve around them whether we like it or not!  I love my computer and use it so often it's natural I will teach my kids to use this very handy tool too!  Here are a couple of sites that I head to when my children fancy a computer session:

Disney Junior: http://www.disneyjunior.ca/en/ An excellent site with games for all ages and abilities
CBeebies (BBC, UK): http://www.bbc.co.uk/cbeebies/  Good for those in the UK
NASA Kids: http://www.nasa.gov/audience/forkids/kidsclub/flash/index.html  It's got Rockets, so it's soooo cool, apparently!
National Geographic Kids- http://kids.nationalgeographic.com/kids/  My son likes to ask questions about everything and this is a good place to direct him when Mummy has run of of useful answers in return!