You've pretty much admitted you ignore the people you EDIT call leftists, so there really isn't much incentive for those people to answer your questions. As your definition of a "leftist" is effectively "anyone I disagree with", that makes me one - and I'm going to answer your question not for your sake, but for the sake of anyone else who might be reading this. Whether you respond to it oir agree with me, I really don't care:
The concept of consent (in this context) is a legal one. Anyone of any age can consent to various things, because consent is nothing more than agreeing with an idea or an action. Legally, however, people below a certain age cannot consent to break the law. This is why child law breakers are legally treated differently than adult law breakers.
A child cannot legally consent to having sex, because in most states, sex is illegal below a certain age. It does not matter whether the "partner" is someone of the child's own age, or an adult, or whether the partner is a pedophile. Having sex with a child is illegal, and even if the child agrees to have sex, that sex is not legally consensual.
There is nothing illegal about changing your gender, whether that change entails surgery or drugs or just a change of name/clothes.
This is why there is no problem with a child consenting to gender reassignment.