I don't mean throw a hissy fit but somebody portrayed you as wanting to teach sex to 6 year olds before teaching them to read!
The most you can come up with is "I will hit back hard on the issues"?  It makes you look guilty just by not fighting back hard.  Relying on the press to call out McCain will not do it.  Yes, they are finally starting to do their job but it will die down soon enough and only the trully well informed are paying attention to that.  The rest have gotten the message from the ad and saw you not really addressing it and will draw their conclusions.
How about some indignation?  Don't go in the mud with them but do show your indignation.  People will support you on this, even those that may not like you.
McCain was on the view where he was called out on this lie and he stood by it.
How about some:
You all have heard about or seen the McCain ad where he accuses me of wanting to teach sex to kidergardeners before teaching them how to read.  This is one of, if not the lowest form of gutter political ads I have seen.  It is outrageous for Senator McCain to suggest that I, a father of 2 small children, along with all the other senators who worked on this bill, want to teach sex to 6 year olds.  We supported this bill to help children (yes 6 year old children) and their families fight against child predators.  For 6 year olds, it would be things a 6 year old can understand, such as "if a stranger touches your private parts, talk to mommy about it."  How can senator McCain use this to get a political advantage and try to scare voters into thinking I want to teach their children sex?  I never thought something like this could come from Senator McCain's camp, but it has.  John, do you stand by this ad with your name on it? Whatever happened to the civil politics based on the issues?  Whatever happened to honorable campaigns?  Changing Washington starts by changing how you get to Washigton, but even by the old Washington standards, this is intolerable.