I read a great article in Psychology Today about a "safe space" in the home with a high-conflict parent: the bathroom!
That's kind of amazing. I was an only child with 2 BPD parents, isolated by distance from any family, not allowed friends, and I spent a LOT of time in the bathroom, dawdling over showers and such. It WAS a safe(er) place. When possible, I kept the lower level or basement of our house (that's where my toys were, and the TV I was to watch, so no one could hear me - being quiet was a must), but obviously had to be in the bedroom vicinity other times of day, and the bathroom WAS a place I'd be left alone. I never realized WHY I was in there or why I didn't want to leave, but even my room was not a safe haven. After beating me, or yelling at me for hours once he was 'caught' leaving bruises on me, dad would come into my room in the wee hours of the morning, crying and begging forgiveness from me, the abused child. Or, he'd come into my room to beat the cat with a newspaper for yowling late at night. And people wonder why I have insomnia, now.
A problem WITH being a kid, is even if there is a safe(r) space in the house, getting to it during a rage is still not possible, often. An adult can and should remove themselves from the rage, and can walk away from another adult both in the current situation, and if need be, life. Kids can't do this, once the rage has started - especially an only child like I was - there were no siblings to diffuse any of the crazy directed at me. Kids with siblings might be able to take refuge if they are willing to 'leave a man behind' and save themselves.
I think a good thing to help kids, age dependent, is to be an adult who will listen, and be trustworthy. I confided in some adults, who did not understand, and it got right back to my parents. This is a big reason I cannot do traditional therapy, now - my trust centers are totally broken. As the kids get a little older and can understand better, let them know they are not broken, they are not bad children, and they do not cause any of the the things in their homes (well, obviously only tell them this about things they did not cause - if they are in trouble for not doing homework, then that is justified). I took the blame for my parents' failings until I was in my 20s. It was a long road letting go of that unearned guilt. And I am still on this site and am still wrong towards it. Let them know things like moms and dads can say things when mad that are not nice, like all people.
"Mother is the name of God in heart and lips of all children" - Byron.
Kids sadly assume parents know all, and have all authority, and in healthy households, this is not bad. But in a household where a parent cannot emotionally regulate themselves, kids often end up being 'parentified' and asked to exert more emotional control than the adult(s).
So letting them have a space to be kids, not adults, is a way to help them, if you can.