A lot you can do. Tell the client that if they do not pay you what is owed thenn youn will file a complaint with the Department of Labor seeking payment of back wages and overtime. This may scare the employer because there is also IRS issues in relation to paying a nanny and whether you are an employee or not. Best you consult with an employment lawyer.
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You did not indicate the terms of the agreement. You also did not indicate how many hours you provided daycare or childcare services. It sound like it was very short term and that you were operating a quasi-business. A nanny usually performs services in the home of another. A daycare or childcare usually provides services in another location. If you believe that the family breached an agreement, you may be able file a small claim lawsuit for the amount you claim that you are owed.