Surely medics have a responsibility to give full information on the outcomes before seeking consent?
One of the known ethical issues in consenting people for a trial is therapeutic optimism . I'm not talking about children here necessarily but adults - so there shouldn't be the extra problems.
Even if the trial recruiter asks for the 'teachback' as a way to check that somebody understands that (say) this Phase 1 trial is not going to have a curative outcome for them because it's just a dose-finding study (for a drug), an adult will give the 'correct' response. But…probing further, alongside this intellectual acceptance, the person has therapeutic optimism that somehow it will be different for them and they will have a curative outcome.
For scenarios such as those who believe they will change sex, they have a ready made supportive network telling them that this is true ('Don't listen to the doctors, they have to tell you that'). This mythology is maintained by ostracising the detransitioners and others who question the narrative.
It's like those groups who genuinely believed that they would live forever. As time went on, and people observed themselves ageing and began to question how this immortality would work, they would be excluded. If reports came in of their deaths, 'They stopped believing'. I've sometimes wondered what happened to this people and if they ever acknowledged the reality around them. (Of course, I could be wrong, and they're all hanging out in Shangri-La with She ).