My husbands ex constantly tries to turn the kids against him: Advice?

I have a question and need some advice. My husband and I have his children several days a week every week, and we do absolutely everything we can to make sure they have everything they need when they are with us and do as much as we can for them, but their mother constantly involves the kids in arguments between her and their father and she constantly posts on Facebook negative things about him even though she cheated on him with his best friend and was also part of the problem and is trying to turn his kids against him what should we do?