I’m NOT Muslim so don’t take this as the final say. But Halloween is a big NO for most Muslims and doing the portraits would be especially bad in the eyes of the American Muslims I know. Many do let their kids dress up and trick or treat so they’re not left out but not all of them do. I highly recommend talking to the parents of the kids beforehand and let them know that you want their kids to feel represented in a way that is respectful of their beliefs and ties to Halloween.