I agree with suki44883's post.
Don't leave your kids alone with any of them and move immed. or have him leave immed.in order 2 provide a
safe + sane + peaceful place for your kids. Listen to your gut, intuition and act.
You are thinking that his kid's mother is not the best, but yet you putting your own kids in harm's way. Take charge of this.
" I think change is scary and I am just heartbroken for my kids"
Fear of change is not a reason to stay in this relationship, if it is not healthy for you and your kids. They need to be the priority.