HELP! IM A NANNY. CHILD SAYS I HIT HIM.

I've been in childcare for almost fifteen years. This recent job was the most difficult by far. There are twin 4 yr old boys who lost their father a few years ago and have MAJOR behavioral issues. They have already been expelled from two daycares and three babysitters quit. One left crying everyday and only lasted a week. The boys hit, spit, scratch, swear like pros, and don't listen to a word they are told. One of them broke their older sisters arm out of anger. I wish I could say I am exhaugerating but I am not. They have a counselor come to the house twice a week to work with them. Yesterday, I put one of the boys in time out for attacking his brother. Time out consists of being in their bedroom in bed while they calm down. During this time out, The boy yanked out all of his dresser drawers and threw the clothes around the room. When I checked on him from the noise, I noticed he had a bruise that resembled a line so I figured it was the drawer and left it at that. I told the mom when she got home about the bruise and that was that. She knows how they are. This morning after i wake up to go back there, she calls and says that the boy told her I smacked him. He even did role playing and showed her what i supposedly did. I am freaking out. I have worked for them for three months and I can't believe I didn't worry about this happening before! The mom said she is going to have the therapist come and try to see if she can get the story. She says she doesnt believe either me or the boy completely. Either way, I am out of a job. I'm freaking out. Any advice or anyone gone through something similar?