There's currently no reason to believe that acupuncture is anything more than a placebo, just like homeopathy (that I've seen anyway).
Here is a good overview, along with links to real studies.
Your childhood experience, rather than showing a lack of a placebo effect like you claim, instead is a stronger case for a psychological response, in that children are more easily focused on other things. An easy explanation for an immediate response, beyond a placebo effect, is endorphins released in response to the needle puncture (which would coincide with the fact that, as the article above mentioned, "sham" acupunctures have results indistinguishable from "real" acupunctures).
EDIT: in response to Miriel.