One difficult realization for parents is that their children will sometimes want the other parent. This can be challenging when there is a divorce because both parents don't live in the same home. You have to remember that this is a trying time for your children, so they might just need some help with the adjustment to having divorced parents.
There are many reasons why your child might want the other parent. For many children, just knowing that they aren't right there is what makes it hard for them to accept. Younger children may be used to knowing that both parents are around and they might not be sure how to navigate the new way.
Make sure that you aren't taking the children wanting the other parent as anything against you. Try not to get down or frustrated about it. Speak to your child and find out what they need. They might not really know what to tell you but trying to work through their emotions with them might be beneficial.
Another reason why some children might cry out for the other parent is that they are kicking back against the changes, including the rules that you've set. When this is the case, you have to gently stand your ground. You can't bend the rules because then the child will always expect that.
It might be possible that just allowing the child to call your ex or do a video chat with them might help the kid to feel better. Discuss the matter with the other parent to determine if this is possible. Having a plan in place for when this happens might make it easier to handle.