If you find out a child is being mistreated, no matter if this is your child or not, it's important to offer some help.
You can do these:
First, take the child away from the people that bring damage to the kid. Call the police if needed;
Then, dial the number of your state's child protection system. If you don't know one, call the Childhelp National Child Abuse Hotline and ask for help. The number goes (1-800) 4-A-Child or (1-800) 422-4453. You can visit its official site as well: https://www.childhelp.org/hotline/
To warp up, do 2 things. One is to keep the child safe. The second is to ask for help from professionals.