Cara just to be clear, the protected characteristic of gender reassignment can apply to under 18s, but under 18s cannot get a Gender Recognition Certificate so can not change their legal sex.
This is what the Equality Act says about single sex services. I've put in bold the relevant parts for loos and changing rooms:
27(1)A person does not contravene section 29, so far as relating to sex discrimination, by providing a service only to persons of one sex if—
(a)any of the conditions in sub-paragraphs (2) to (7) is satisfied, and
(b)the limited provision is a proportionate means of achieving a legitimate aim.
(2)The condition is that only persons of that sex have need of the service.
(3)The condition is that—
(a)the service is also provided jointly for persons of both sexes, and
(b)the service would be insufficiently effective were it only to be provided jointly.
(4)The condition is that—
(a)a joint service for persons of both sexes would be less effective, and
(b)the extent to which the service is required by persons of each sex makes it not reasonably practicable to provide separate services.
(5)The condition is that the service is provided at a place which is, or is part of—
(a)a hospital, or
(b)another establishment for persons requiring special care, supervision or attention.
(6)The condition is that—
(a)the service is provided for, or is likely to be used by, two or more persons at the same time, and
(b)the circumstances are such that a person of one sex might reasonably object to the presence of a person of the opposite sex.
(7)The condition is that—
(a)there is likely to be physical contact between a person (A) to whom the service is provided and another person (B), and
(b)B might reasonably object if A were not of the same sex as B.
(8)This paragraph applies to a person exercising a public function in relation to the provision of a service as it applies to the person providing the service.
Separate schools legislation also requires toilets to be single sex:
Regulation 4 — (1) Subject to paragraph (3), suitable toilet and washing facilities must be provided for the sole use of pupils.
(2) Separate toilet facilities for boys and girls aged 8 years or over must be provided except where the toilet facility is provided in a room that can be secured from the
inside and that is intended for use by one pupil at a time.
(3) Where separate facilities are provided under paragraph (1) for pupils who are
disabled, they may also be used by other pupils, teachers and others employed at the school, and visitors, whether or not they are disabled.
(4) Suitable changing accommodation and showers must be provided for pupils aged
11 years or over at the start of the school year who receive physical education.