Children are usually excellent at adapting to changes. This might not be so true when their parents are divorcing. This is such a major life upheaval that even the most adaptable children might find it challenging. As a parent, your job is to try to help them get through this.
One thing that your children might be worried about is that they will have to choose between parents. They need to know that this isn't how things will work now. In order to combat this mode of thinking, you and your ex might need to sit down together with the kids to talk to them about how the situation is going to be handled. If possible, have the parenting plan with you when you do this so that you can share the pertinent information from it with them.
Being able to have a group meeting like this also sets the stage for a united parenting front, which can also provide stability for the children. It shows them that they can't play one parent against the other because you are working together.
While you are speaking to the kids, find out what is bothering them. Let them talk to you about the situation. They should be respectful, but they should be free to say how they truly feel. You might have to help them find suitable ways to communicate their feelings.
Some children might be upset because they feel like everything is changing and they don't have a say in anything. Help them to find the things that they can control so that they can focus on those instead of worrying about what they can't control. Ultimately, this can all help them to thrive in the new way of life.