Children often fight back against what their parents say is best for them. Testing the boundaries is a normal part of being a kid. When their parents are divorced, there is a chance that they will take this to another level. They might try to play one parent against the other in an attempt to get their way.
Both parents have to combat this. Be vigilant about watching for signs, such as the kids running to one parent when they want to do things or trying to keep you from speaking to each other. Regardless of what is going on with the kids, they need to know that this is unacceptable behavior.
Discuss major decisions
One of the best ways that parents can avoid the kids trying to get one parent to agree to something the other parent has forbidden is to discuss all major decisions. Kids might try this for smaller things, such as bedtime changes, but the bigger issues, such as trips with friends, might need to be the focus. You and your ex should determine what decisions need to be made jointly and stick to that plan.
Don't encroach on each other
Each home may have slightly different rules. Don't try to force your ex to do things like you do at your home. They shouldn't expect you to run your house like they do theirs. Keeping things divided in this manner gives each adult the freedom to do things in the way that makes them comfortable. It also gives the kids some flexibility so that they don't think they need to manipulate their parents.
Keep tension to a minimum
Kids will often prey on tension to get what they want. When they know a parent is at the end of their rope, they might push a little bit in the hope that the adult will cave. You must stand firm with your decisions even when you are exhausted so that there isn't a precedent set that is difficult to overcome. Finding ways to work through conflicts with your ex is beneficial. Additionally, remember that all discussions about contentious matters should happen without the kids present.
You can also set standards for ground rules and conflict resolution in the parenting plan. This ensures that you and your ex are on the same page with these major child custody components.