I'm literally just standing in front of a door before someone else
Is there something you're not telling me? What is going on near the door?
Yes, I'm frightened by them beating the boy inside the house.
Being in the full view of abuse is pretty scary. Yell or make noise so that they stop, then threaten to call the police!
I don't want to get into trouble with the police, I can handle it.
I hope you will call the police. This is a serious matter and the police are better equipped to handle it than a citizen.