Fear.

Anti-Hitlerism is the "religion" of the day. To signal against it makes you a heretic or blasphemer, so the parents or people around said person are reacting from fear they experience perceiving that they are in danger. Which they are, socially, financially(employment) and so on.

Fear of reprisal and consequence motivates them.

They pipe down and take a knee when they themselves are victimized, believing that if they are compliant and agreeable they won't upset the apple cart and will remain "safe" rather than have the courage to stand up for themselves or their loved ones against the mob that they are afraid of.

When fear dominates your soul, this is what your life becomes.

Slavery.