I don't even accept the concept of 'genuinely trans' so accusations of transphobia mean nothing to me.
Dysphoria is, by definition, a mental health issue. The clue is in the prefix 'DYS'.
You don't treat mental health issues by blocking healthy, normal natural bodily processes, or by giving cross sex hormones - to someone whose natural biology is not programmed to be receptive to it without it causing harms.
There is a reason why the child is feeling dysphoric or distressed. That is best explored with appropriate therapy.....and that is certainly not 'trans affirmative' therapy. No child is the 'wrong sex'; they simply are the sex that they are and the most appropriate therapy will help them resolve their dysphoria, or certainly cast some light onto what has caused it.
Your child is just as the relative start of their adult life.......given 10 years, or maybe even fewer, you may very find they desist. Most do; even those who have gone through surgeries such as mastectomies.