I usually say, “That’s a good question. What do you think?”
Whatever their response - usually some variety of “A boy because you have short hair,” I tell them I am a girl because I feel it is more important for me to be a positive example of another way a girl can be than it is for me to explain gender identity to a 7 year old.
I am a school bus driver and I tend to get this question about once a month.
What do you do, or would you do in such a situation?