Well, you still have to let your kid be a kid. Its no use letting it hear Steve Vai or something when he/she is 4 years old.
You can say 'hey, check this out', but you never can influence somebody's taste. Or let me rephrase... you never SHOULD influence it. He'll probably rebel against it as a teenager and start wearing golden chains!
But - and this is NO joke
- when my (boy) kid turns gothic, I kick him out of the house! He can listen to lotsa stuff, but gothic... no!