I think your plan is good, give a child input that he enjoys and the nuts and bolts of Korean will be much easier later on.
I think the site ' http://study.korean.net
' (use internet explorer) is great for kids.
Lessons, games, kids songs, comics and fairy tales all written for Korean learners. Some of the material might be aimed for children that are a little older, but I'm sure you'll be able to find something there (free registration required for some features)
has some good storybooks with audio, but for full features I think you need to be a Korean to sign up (my Alien registration number didn't work). I can't really figure the site out too much.
http://kr.infant.kids.yahoo.com/infantz ... tmain.html
have a bunch of other fairy tales (they share a lot of the same ones)
Some of them have on screen Korean subtitles, but not all.
I recommend this series of stories:
There are no transcripts, but the stories are original and fun and I think your son will pick up a lot just out of the pictures and context.
*if you're specifically looking for audio you can use a program like http://audacity.sourceforge.net/
to record the songs/ fairy tales directly from your computer speakers into mp3 form (just set your microphone to 'stereo input' and press record)