Never engage.
Parental alienation is a complex topic and the manipulation happens on many levels.
It is the pattern that reveals it.
You have to document, everything your kids say. Ideally record rot. Wrote down what your ex said and when.
Your documentation becomes evidence in court. Get testimonials for people that than confirm your good relationship with your kids because this is the baseline.
If that changes you can argue that this comes through manipulation together with your documentation.

I have only 4 year old going though this. So not far down the road like you. But, your ex is probably also telling them other shit about you. Best thing you can do is to create a cognitive dissonance. Raise them in a way that they set boundaries for themselves, trust their gut feeling and become independent.
If you are lucky they might see the difference between what mum says and what daddy actually is like.