What does atticus mean when he says “ they’ve done it before and they did it , tonight and they’ll do it again and when they do it seems that only children weep” ?

it probably mean what they've done before they will do again making it seem as grown men and women are far more 'superior' and more fearless and courageous letting them look down on their own child self think man i had it easy and yet i still complained when i was little.

hope this helps and this is just how i see it  ;)

Children do not really accept it and not fully understand the concept of prejudice and how a black man's word against a white man's word is a no-brainer and the white man will automatically win.

Explanation:

citizens are given 25 years of life to prove their worth to society; if they fail to do great things