After you marry, you can create an agreement that irons out certain aspects in the event you decide to divorce. Many people will decide to create a postnuptial agreement when they have a big inheritance or if they start a business. It is protection for both parties.
However, this type of marital agreement must follow the law to be valid. According to the New York City Bar, there are some things you may not be able to include or that you must be careful about including, such as child support.
You can include something about child support for children that belong to you and your spouse, but you should expect the court to review it. A judge can throw it out. Child support decisions must be in the best interest of the child and require deciding at the time of a divorce.
You cannot exclude yourself from providing support to children you have a legal obligation to provide for financially. If you include this in the agreement, the court will throw it out.
If your spouse brought children into the marriage and you do not adopt them, then you can include provisions for the support of them in your postnuptial agreement. You do not have a legal obligation to provide for them after your marriage ends, so the court will not scrutinize any agreement about support. However, you can agree to provide for them financially should you divorce when they are still underage.
In general, including child support in a marital agreement is not the best idea because a court can change it for children you have a legal obligation to support, but it can be a good idea for children who are not legally yours.