Many parents who go through child custody cases soon find that the children are doing things with one parent that they know the other won't allow. There isn't anything necessarily wrong with this unless the children are playing one parent against the other, which is a problematic. You and your ex might have to work to ensure that this isn't happening.
There are several things that might signal that your children are trying to play you against your ex just so they can get their own way. One of the most common is that the kids will go to each adult separately. If the first one doesn't give them the answer they want, they might change up their plea a bit so that they have a chance of getting the other one to agree.
Having open lines of communication with your ex can help in these cases. Even if it is a just a text letting that parent know what you told the children, you might find that it helps to curb the child trying to work on both parents. It might also benefit the situation if you explain that manipulation isn't acceptable behavior.
It can also help if you have a set plan for handling decisions related to the children. Your parenting plan can cover things like curfew and guidelines for homework. The more details you have in these, the less likely it is going to be that the children will be able to get away with trying to get one parent to agree to something that the other parent has already said no to.