Children cannot get a GRC so will not have the protected characteristic of gender reassignment.
The protected characteristic is broader than that:
A person has the protected characteristic of gender reassignment if the person is proposing to undergo, is undergoing or has undergone a process (or part of a process) for the purpose of reassigning the person's sex by changing physiological or other attributes of sex.
I'm not a lawyer, but I think that would cover a child who was taking puberty blockers and/or cross-sex hormones. Maybe a name change and a hairstyle would not be enough though.