Literally all of those arguments or questions are answered in the trial protocol. Have you read it? Or even just the study synopsis? It is publicly available on the link I posted previously and will describe the aim, possible benefit, consent process, risk balance etc.
There is a legal distinction between consent and assent, so it very literally does change that. You can't just make statements and say they're fact, that isn't how facts work.
The drug(s) are used in practice for various indications. This isn't a study to primarily investigate what the drug does (although it is part of the secondary outcome), it is a study to investigate the risks and benefits of a therapy that has been and is used, and that many people want to be able to access.
One of the inclusion criteria is literally that the child/young person has participated in other forms of care for their holistic health. Another is that they must have a diagnosis of gender incongruence. They also must be thoroughly counselled on the risks, multiple times.
And finally, all research that reaches participants is ethically approved. An ethics panel will have reviewed this in great detail and have agreed it can proceed. Why would your opinion (based on, as I assume, not even reading the protocol) trump that formal process?