A child witness might not be competent to testify. Some states have age limits, some allow a judge to question a child to determine competency.
Polygraph exams are normally not accepted sufficiently in the scientific community and not admissible. I know of no "tests" that are admissible to determine truth like what you are describing. The examiner would be offering an opinion on credibility, which normally is NEVER allowed. Credibility is for the jury to decide.