Give accurate information, and don't present suppositions as facts.
Then layman shouldn't challenge the exhibits unless they understand the scientific method enough to form an intelligent and relevant challenge. A tour group is not the place for seminar caller challenges.
If a person has a problem with the exhibit and would like to challenge the presentation, the proper venue is talk to the person in charge of the exhibit or to write the museum or park a letter asking for clarification.
Questions on a tour are fine - just not seminar caller questions and not ones that aren't meant to be questions but challenges.
Give accurate information, and don't present suppositions as facts.
The problem is that a scientific theory is not supposition.