Mermaids is a registered UK charity which exists to support families with a child identifying as transgender or having some doubts about gender. It used to be a very small support group. It is now run by Susie Green, whose eldest child, Jackie, is a transwoman. SG took J to New York when he was 12 so that a doctor there could prescribe cross-hormones for him. This would not have been possible in the UK at the time. She took J to Thailand for sex reassignment surgery which took place on J's 16th birthday. This would also not have been possible here at the time and the Thai government has since changed the law so that it isn't possible there either.
Under SG's management Mermaids has grown a lot and has received grants from Children in Need amongst others. It does a lot of training in schools and other public organisations. SG thinks the Tavistock Gender Identity Disorder Service is too conservative in its approach to putting children on puberty blockers and cross-hormones and there should be minimal gatekeeping. (Former Tavistock staff have reported that Mermaids staff/volunteers are known to be supporting children/parents pushing for faster access to puberty blockers/cross-hormones.)
She has tweeted approval of the North American approach of allowing minor children to consent to surgery, e.g. double mastectomy in the early teens. She believes strongly in affirming a child's stated gender identity without question, and if that means getting them onto a medical pathway early, so be it. (She has no medical, psychological or other relevant professional qualifications at all, by the way.)
SG presided over Mermaids at a time when it had a major data breach - last year? I haven't seen any report of the outcome of the investigation by the Data Commissioner on that. SG's professional background was as an IT manager for the Citizens' Advice Bureau, so this was particularly surprising.
Surprisingly for a very high profile charity growing fast, Mermaids has also had extended periods recently when it had very few Trustees. Its website has also sometimes indicated that some of their Trustees have opted not to have their details published. This seems odd, but I don't know about charity law. Maybe there's a good reason.
As linked above, Mermaids was also criticised in a legal case concerning a very young boy whose mother was trying to socially transition him against his father's wishes, but with the enthusiastic support of Mermaids. The judge sided with the father.
In recent weeks there are signs, however, that some transactivists are turning on SG and Mermaids. They believe all trans groups should be led by trans peope and SG isn't trans. So, watch this space!