Just a thought... It seems that the "How did YOU so that?" response comes up more when the presentation is more centered on the magician. If the presentation involves the spectator there is less emphasis on the magician's skill. Also, I think that a logical, well written script also helps avoid that question. If your presentation is focused on what you are doing in the trick, the question seems to arise more.
I do like the "I'm glad you liked it" response. I also agree with Christopher, in that I usually dont respond unless they ask a second time.
One additional consideration is your character. Whatever response you provide has to be consistent with your character.
Rick:
Your story reminds me of when I was doing a Cub Scout show and I ask someone to pick a card. A kid yells out loudly, "I know this one." The whole room looked at him, so I couldnt ignore it. I stoped what I was doing and acted dejected. "Of all the card tricks in the world, I had to pick one that you know." I then act like I'm thinking. "OK, I'll do a different one." I turn to the spectator, spread the deck and say, "Can you pick a card, but not he same one you were going to pick before..."
As you know, with kids the I Know This One is more of a general sense of familiarity expressing that they have seen something similar. As anyone with kids know, they like to see things that are familiar (which is why they watch TV shows and movies multiple times). The kids I Know How You Do It is alsoa different response than adults. Chances are they dont know, but you want to keep them from going any further because every other kid will want to take a guess. The best response there is to encourage them to tell you after the show. For my kids show character, I can get away with saying, "I've always wanted to know, maybe you can tell me after the show." Younger kids forget about it, because they really dont know. If an older kid does know, they usually have an interest in magic and I'm glad to talk to them after the show.