Take the current massive wave of attempts to legislate puberty blockers as illegal (or even taking your kid to a different state for this purpose). On the face of it "people should wait until they're grown up" sounds reasonable.
What's missing is that puberty blockers actually just delay irreversible / only surgically reversible changes, things like breast growth, facial hair and vocal changes.
Forcing a trans kid through the wrong set of changes is a massive driver of depression and suicide, not just during puberty, but also later in life. They feel broken (hell, _I_ feel broken). Fixing these permanent changes is often extremely costly and care is not easy to access, so everyone else needs to exist in a state that could be described as "uncanny valley" to most people. Mental health outcomes in trans people are directly correlated to how soon you stop this process. In essentially all cases (more than 99% of all cases in the UK) these children will not change their mind either, so in essence we have already compromised on giving children hormone replacement during their puberty for the miniscule chance of hurting a theoretical "confused cis kid".
Cis lives matter more than trans lives in this system by a factor greater than 1:99.