Currently, very young children haven't been around long enough that the environment and their nurturing can account for the fact that some of them have the brain structure of the opposite chromosomal sex.
Even if epigenetics can account for some of a young child's gender dysphoria, it is not the child's fault in this life that the child has gender dysphoria. Neither is it the child's responsibility to alter the child's brain structure to conform with the child's chromosomal sex. It is also not the responsibility, or even the unilateral right, of adults to decide to alter the child's brain structure to conform with the child's chromosomal sex.
Furthermore, as children age, they do not lose their rights.