Child custody matters don't bring out the best in people, so remembering that things might get a bit difficult as you venture through this may help you to address the issues. One thing that shouldn't ever be acceptable as you handle these disagreements is either parent badmouthing the other.
Having to deal with your co-parent saying negative things about you is a challenging situation because you can't just ignore it. There is a chance that they're doing this in front of the children, which could have an impact on how your children interact with you. Even if they aren't saying harmful things around the children, the kids might still find out about them.
It's a good idea to have terms in the parenting plan that forbid this type of behavior. The basis of the co-parenting relationship must be mutual respect. If either parent isn't abiding by this, it can harm the children because they may have to deal with the stressful situations that come from it.
Instead of playing into the drama that your ex wants to cause, you can focus on making sure that your children have what they need. This isn't likely going to be easy, but you may find that it helps you out.
The children must remain at the center of the parenting plan. It is especially important that you act if you feel they are in danger. You may need to turn to the court or use an alternative dispute method to get things situated. You don't need your ex to be able to show that you retaliated in any manner.
If your ex-spouse is making co-parenting impossible, find out what legal options you may have that can enforce your parenting plan.