It sounds to me like testing - both to see whether SoberLink really will detect alcohol in minute amounts (yes) and to see what you'll do if he blows a positive that is a tiny, tiny, positive, right before something that your kid has been looking forward to.
But 0.00 is 0.00, and anything else is not 0.00. So you did the right thing by nixing the movie.
If your kid knows about SoberLink, I think it's okay to tell him "Dad didn't provide a clear test, so unfortunately he had to cancel the movie". This is Dad's decision and responsibility, not yours, and certainly not your child's.