I feel oddly sympathetic with your situation even though my experience was the other way around. My stepmom would bully me every other weekend together with her kids. Or they all decided to ignore me and not speak to me and then complain to my dad so he'd scold me. My dad was usually away and didn't do anything about it even when I begged him to make the situation better for me (at age 7 I had to beg him to stop his wife and kids from terrorizing me). He refused to do anything about it and I walked away from all of them a few years later.
Anyway, it taught me that being passive is just as harmful. Your husband probably doesn't see/understand it that way (yet) but hopefully you can give him insight in how it is for you and him not doing anything is just as bad as what the kids are doing . And maybe in the end even worse since you picked him for a life partner so you need to be able to depend on him. He has his kids but he also chose to marry you. It's his job as a father to make sure his kids aren't terrorizing little shits. And as a husband that you feel good in your own home.
Anyway, it taught me that being passive is just as harmful. Your husband probably doesn't see/understand it that way (yet) but hopefully you can give him insight in how it is for you and him not doing anything is just as bad as what the kids are doing . And maybe in the end even worse since you picked him for a life partner so you need to be able to depend on him. He has his kids but he also chose to marry you. It's his job as a father to make sure his kids aren't terrorizing little shits. And as a husband that you feel good in your own home.