Flora As my child was well into puberty we unfortunately were too late for the puberty blockers but I have researched them as part of my continuing learning curve on this (before last year) unfamiliar path.
As a former surgical nurse, who actually worked with the first M>F realignment in a major London hospital many moons ago I like to know facts as a) things have changed SO much since then & b) this was/is my child that needed my support which clouded my previous objectivity somewhat as you can imagine! I only say this to show where I am coming from. 
The current research shows that puberty blockers are totally reversible, a pause button on the development of the secondary sexual characteristics that are so abhorrent to the child. These are not started until the last possible moment before puberty in the UK with massive amounts of psychological support and counselling before starting for both the child and family (forgive me if you are aware of this but it seems a lot of posters are unfamiliar with the process so doesn't hurt to repeat)
The thing that would have swayed me is that future fertility would be nil if my child committed suicide, which was a real and awful possibility. The rate of suicide attempts is approx 70% in trans teens who are not supported by parents and don't get appropriate counselling and treatment. God knows how many are succesful but it is thought to be a good 40%. The average life expectancy of any trans person is 23 years old due to the high suicide rate and higher risk of murder. (1 in 12)
Each case has to be judged on it's merits, and while possible side effects of any treatment on future fertility are part of the discussion, it would not be a deal breaker if it kept my child alive and they understood the risks.