In the United Kingdom, transgender minors are treated by the Gender Identity Development Service, a part of the Tavistock and Portman NHS Trust. (GIDS is a part of NHS England, and its offices are all in England, but it treats trans minors from throughout the UK.) GIDS does not provide any surgical interventions, and patients under sixteen are only offered GnRH analogues, which prevent puberty. Until early this year, even this was only available to patients who were at least twelve years old, but that policy was changed to allow under-twelves who have begun puberty, and display Gillick competence (a longstanding standard for determining whether a person under the age of sixteen is able to consent to medical care), to stop it. (GIDS does not accept parental consent to prescribe GnRH analogues for patients who do not meet the Gillick standard.)
The use of GnRH analogues for puberty blocking is fairly well-studied. While the majority of research is on cis children experiencing precocious puberty, some studies have also been done on trans children (and trans adults who used GnRH analogues as children). Of course, as always, it is not totally risk-free — there is a chance of reduced bone density, as well as of episodes of physical discomfort — but this risk must be balanced against the harms of a gender-dysphoric puberty, both psychologically and in terms of later necessitating more invasive treatments to reverse its effects. Puberty blocking does not cause irreversible changes; the medication must be taken until starting hormone supplements, or puberty will simply continue as typical, though delayed.