(Late to the thread) Some of the responses here are absolutely disgusting. Coming from a transgender male, the best thing you can do is support her. Let her cut her hair, wear what clothes she wants, refer to her as whatever she wishes, get her counselling.
Absolutely no way in hell that she'll be on any medication 'in no time'! I know many people who have had many different experiences and I've been through it myself and have seen multiple therapists. It's not an easy journey. I wish I had figured myself out at 13! Nobody gets it easy.
Your child also will not be making any 'permanent changes' to her body, that is absolutely ridiculous and comical that people assume such a thing.
She'll be put through years of therapy and see many people about this, the first medication she will get are called 'blockers' and they block the body from producing hormones she doesn't want, basically pausing puberty. Completely reversible.
Then when she turns 16 it'll be legal to be put on testosterone, which she'll have to inject for the rest of her life, but that does not mean she'll be offered it a week after her birthday or anything.
You have to be 18 for any surgery.
If your child gets turned down after she's 18 but still insists that she transitions then it'll be out of your hands and solely her decision.
Did you know that about 40% of transgender teens attempt suicide? Would the ignorant people in this thread rather have a dead child or a trans child? I really don't understand why you would have children if you wouldn't be willing to love them if they turned out to be different.
There's scientific evidence stating that transgender individuals brains are more similar to the gender they identify with than their birth sex. Gender dysphoria* being considered a mental illness is a controversial topic, but I would say that yes, it's a mental illness. And the cure is therapy and hormones. If you're honest about everything and don't rush, then honestly there is only a slim chance that your child will regret anything. Children will most likely grow out of the phase before any permanent changes are made. Also- chest binding* is not dangerous if done safely, the only way it can cause harm is if you deny your child and won't allow them to bind, therefore the child results to drastic measures I.e using tape or bandages to flatten their chest.
Its 2017, the new generation is far more accepting than the previous. Yes, there will be bullies, but don't think for a second that your child will be completely isolated. The internet is probably the only place she knows she can be herself and even has friends who accept and support her, please don't ever take that away.