What’s it called?

Gaslighting?

I did wonder if it was that but I wasn’t sure because it’s said to my child

It's called toxic. Undermining your authority in front of your child. It's also dismissive of your thoughts and feelings.

My husband and I have done that from time
To time. It’s really bad, we’re doing it to get at each other. I feel awful about it. But I’m not sure it’s that exceptional is it? I wouldn’t end your relationship over it or try and seek out a label for it. Maybe it’s just a stupid thing to say?

Deffo not gaslighting!!!!

It’s called parental alienation and is a strategy whereby one parent intentionally displays to the child unjustified negativity aimed at the other parent. The purpose of this strategy is to damage the child's relationship with the other parent and to turn the child's emotions against that other parent. It’s emotionally abusive to the child.

Gas lighting, deflection, attention seeking (as if he’s the child),
Er…my x used our daughter as a human shield, held her while close, like a cuddle while saying similar things but really he’s the one picking arguments n fights with me. I used to settle or do things just so I didn’t get one.
Unfortunately it’s so the child thinks bad of you n better of them.
Don’t want to tell you what to do but it doesn’t sound good or healthy for either u n child.
I know easier said than done, but if he continues and can not see the error, I would try n get away as soon as you can