In my opinion, there is nothing wrong with how you handled the question. I believe that if kids are asking the question, they are ready to hear the answer. In most cases, I would say a 6 year old wouldn't require a more detailed answer than the one you gave, however
, since he is asking for more detail, he needs to have the information. Better coming from you in the correct fashion than from peers, which may or may not be correct and appropriate.
In my case, my daughter was 6 when she asked where babies came from. My answer was generally like your response. When she was 9, then came the questions of exactly how does daddy's sperm get to mommy's egg? That question required a much more anatomically detailed answer.
I believe in a lifelong conversation when it comes to talking about sex with both my kids. I have a couple of books, and I am always available to answer questions and sometimes I initiate the conversations myself when I feel it's warranted. The conversations righ now with my 14 year old daughter are dealing more with boy/girl relationships, expectations of treatment by boys, birth control, sexually transmitted diseases, teenage pregnancy, etc.
I would much rather handle these topics myself rather than leaving it up to their peers and/or the public schools.