I think teaching personal safety and consent can go a long way to teaching a child what is appropriate vs inappropriate behavior. I think it also helps to create an environment where children feel safe telling us about scary things that happen to them. Otherwise, we mostly have to stay alert for signs that our children are in an actual unsafe position with the other parent.
But when they’re in our homes, we can make every effort to only speak in positive ways without directly lying about anything. I save my complaining about my exes for when my kids aren’t around to hear it. Otherwise, I try to foster a healthy relationship and assure them that it’s not a competition and they don’t have to choose. I want them to feel equally at home in both houses without feeling pulled between them. I can’t control what happens there, but I can certainly do my best to encourage healthy relationships on my end.