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:

brutus overlooks that the majority of the crowd served julius caesar the military general. he appeals to their citizenship as romans by calling them first by that name in his salutation: "romans, countrymen, lovers." mark antony, however, speaks in terms understood by the military, by calling them first by, "friends, romans, countrymen." he recalls not only caesar the general but caesar the friend, whose body they see covered with stab wounds.