It's a hard one. I put up with alot of comments directed toward me when I was a child by my mum, it's the comments I remember. She put herself first, not me, her husband at the time (my abusive stepfather) came before me too. Having a bp mum was hard growing up, even though I didn't really know why.

However, despite my traumatic childhood, I've turned into a decent grown-up.
I think it can go either way with children, they either follow the same path as their destructive parents or go the other way and have a decent life.
I'm a better parent because of my childhood. I know I am a good mum.

Maybe when she starts on your son, you could take him out to the shops or something, just remove him from the situation.
Talk to your son, see how he feels - no one ever thought to ask me how I felt about anything going on in my life when i was a child - if it is disturbing him alot, maybe you need to keep your children safe over your relationship.

I agree with Avalanche but also you need some family therapy. for you, the kids and your partner. sometimes anything is easier to handle when you have someone to talk to about it. and a therapist can tell you what kind of affect this is having on your kids.