My kids were a bit older when I left their dad (youngest was 8), but I've always been upfront but respectful of their dad with them. I told them that their father had a disease called alcoholism that made it so that when he starts drinking, he can't stop -- and when he drinks... and here they filled in "he's mean" (I didn't even have to say it).
I told them that until such a time as he decided he wanted to get help, go to a doctor, it wasn't safe for us to be around him. I told them it's OK to be sad about that, it's OK to love him and miss him, but that as their mother, I had to put their safety first.
I don't know if any of that helps? I also got them into counseling -- partly because our leaving happened in a very traumatic way, but also because I wanted them to have a safe other adult to vent to, say all those things that they didn't want to say to me -- someone they could tell if they missed their dad, if they thought I was being mean, if they had questions they didn't want to ask me...