What is the law of having a boy and a girl share the same bedroom, the girl is 10 years old and the boy is 8 years old? I live in the New York State.

Are they brother and sister ? Are they staying in the room full time together or just on weekends or visits ?

First there is no "LAW" there will be rules that family services go by, so is there a chance someone is going to report you to CPS ( or what your service is called where you are at)

Is there custody issues, will or maybe will another parent use this as grounds to ask for change in custody ?

While it is not a law, it is possible that CPS will look at it negative. Esp as they start getting older. And a custody battle can use this as reasons also.

So what is your specific worry ?