Usually, child support is paid starting from the date of the divorce filing and is not paid retroactively to the date of separation. If you and your spouse are separating, or if you anticipate a divorce in the future, it is crucial to communicate with your spouse and figure out child support sooner rather than later. This is especially true if one parent is a stay-at-home parent, or if one of you moves out.
If your spouse does not want to commit to child support payments or does not follow through on their commitment, you should file for divorce, or possibly even file an emergency motion. The courts generally respond quickly to emergency motions, and you will often get a court date in as soon as two to four weeks. Even in the greater New York City area, they should see you within a month or so.
Ultimately, if your spouse is moving out or you have limited income, you want to ensure you have confirmed financial arrangements. If you are not fully confident that your spouse will follow through, hire a divorce attorney and be ready to move forward with an emergency filing.