I would be very supportive to the child, even discussing at length to ascertain it was not just a passing phase. I have a very gifted 12 year old girl in my charge and though she definitely isn't into changing genders, talks with quite some insight into a variety of subjects including sexuality. I've said that if I don't have an answer, then I'll find out and she's happy with that.
Most 12 year olds are experiencing all kinds of new changes in their bodies and in their feelings. Furthermore, they often feel misunderstood as they are struggling to leave behind their childhood and become teenagers and then finally, adults.
Ahead lies sibling rivalry, pressure from peers, issues such as rebellious behaviour and problems at school, and negative attitudes from others. Other girls her age have been highly critical of her being a 'know-all', stuff like that which angered me. I would set aside lots of time to listen to my charge. When she asks questions, deal with one question at a time. Mine suffers huge abandonment issues, so this is where I'm being assuring that I'm not going to leave her.