I'm very much for a clinical trial on one proviso: it needs to be ethical and safe.
If the only way to achieve this is to use retro data, so be it. For example, they could do IQ tests, brain scans and bone density tests on the previous cohorts now, then map them against averages for their age. Obviously it's not as good as getting an actual baseline data for each individual child, then checking it throughout the trial, but it's better than risking brain damage on someone who is yet to take puberty blockers.
Presumably any trial would need to track these things over a number of years, so retro data plotted against averages for ages could be useful e.g. a child who had puberty blockers from the age of 11-18, ending in 2020 would now be 22. There are several data points that could be captured here and used as part of the data analysis, such as age first started, number of years on blockers, age when blockers finished.
If instead they are accepting new patients, the ethics of selecting the right ones are a minefield, as stated above by several PPs. Informed consent for possible permanent brain damage is presumably impossible (also there is highly likely to be loss of fertility and possible/likely osteoporosis).
So yes, trials and evidence gathering is the way forward. But, to put it bluntly, that's also what Mengele did. Apparently his data gathering and analysis skills were excellent. However, it's only viable if it's ethical and safe (which Mengele's experiments clearly weren't).