I'm in fear for my kids safety help ?

My 6 yearold step son is a bully and threatens and hurts my children 5 year old girl and 3 year boy :(. We only have him on everyother weekend but I'm just upset when he's here bc I'm afraid now he will beat and hurt my kids. We also have a newborn here too and his violent behavior has scared me to the point of not letting my kids stay here at night anymore when we have him. I'm gonna watch him like a hawk when he is here I just need help and advice. My husband is my life he is the best man in the universe to me but unless we get his son help I'm in fear for my children and our babies safety.