Try being gentle in your assertiveness.
Christians somehow think being assertive is a bad idea but I think the problem is more about how you deliver your assertiveness. Most people use a rude or angry tone. Try being gentle. With a former spouse you might calmly say, “Please stop sharing your opinions about my family with our child. You’re putting him in the middle and I know you don’t want to hurt him. Thanks.” It’s okay to be assertive—even Jesus was assertive. Just take out the rude and put in the gentle. In love, speak the truth.