You're conflating gender and sex. But having a binary only option for this forces non binary children to comply to the gender dichotomy while having an option

You're conflating gender and sex. But having a binary only option for this forces non binary children to comply to the gender dichotomy while having an option outside this norm allows them the freedom of choice. Also, ask yourself why a binary-only option doesn't strike you as "advocating sex change" when the users are capable of choosing different options.