|Dolphins are smart!|
First, there is always the possibility that the child is right. It's not a probability mind you, but it is possible. The likely reality is that the child is wrong, and is still alive merely as a result of your kindness - and weak justifiable homicide laws.
Still, the adult should respond in an adult manner. Stuffing the little snot heads-down into the nearest recycling container is not, per se, an adult response. (Note to self: Write a blog on effective methods of relieving tension; find photo of recycle bin.)
Neither should one engage in debate. This sounds easy, but a response such as, "No, actually I'm doing it right." will surely result in "Well, I was told by Albert Einstein, my mom, and three clever dolphins that you're wrong." You can wriggle around Einstein and the dolphins, but you can't diss mom.
When the little belt tries to correct you, refer back to your instructor. "Well, Master Out-Ranks-Us-Both taught me to do things this way, and this is the way I shall teach you until he corrects me." is a completely defensible reply.
Additionally, as Henry Kissinger would say, "It has the added virtue of being true."
Sometimes the "you're doing it wrong" comment comes from an adult of lower rank, or a fellow student, or (gasp) a senior belt. By and large the response above still works. If you believe that what you're doing is as it was taught to you by your instructor, then you can politely say, "I believe I was taught this method by my Master."
As always, be conscious of your confrontations, i.e. pick your spots. Arguing with Grand Master Kim will never yield success. I have personally witnessed my Masters get something wrong, something I know they know is incorrect - but at the moment - for whatever reasons, they get it wrong.
Let it go; or as my Master once said, you now know two ways to do 'X'. Be respectful; don't argue, pick your spots, refer to your Master.