Why do you think Cole's mother did not tell the circle how his father would beat up on him?

In my opinion, Cole's mother chose to stay silent because she was afraid. Possibly, she believed that she would be beaten as a result.

Cole’s mother dusted imaginary specks off her dress, then picked up the feather hesitantly. She opened her mouth to speak, but a quick glance at her husband brought a frightened look to her eyes. She quickly passed the feather on to Garvey without saying anything.