Special treatment of some group always (almost always, children and some other groups are treated differently and it feels OK) triggers the feeling of unfairness and raises concerns about objectivity of testing process.

> You're only human, and we all are susceptible to misjudgment, stereotyping, etc.

Yes, double-blind trials were invented to prevent an experimenter from nudging results in a desired direction. In the case of increasing diversity, the desired direction is explicitly specified and encouraged.